diff --git a/ompi/group/group_plist.c b/ompi/group/group_plist.c
index 29a969bf85..0f17422e22 100644
--- a/ompi/group/group_plist.c
+++ b/ompi/group/group_plist.c
@@ -29,6 +29,34 @@
 
 #include <math.h>
 
+static int ompi_group_dense_overlap (ompi_group_t *group1, ompi_group_t *group2, opal_bitmap_t *bitmap)
+{
+    ompi_proc_t *proc1_pointer, *proc2_pointer;
+    int rc, overlap_count;
+
+    overlap_count = 0;
+
+    for (int proc1 = 0 ; proc1 < group1->grp_proc_count ; ++proc1) {
+        proc1_pointer = ompi_group_get_proc_ptr_raw (group1, proc1);
+
+        /* check to see if this proc is in group2 */
+        for (int proc2 = 0 ; proc2 < group2->grp_proc_count ; ++proc2) {
+            proc2_pointer = ompi_group_get_proc_ptr_raw (group2, proc2);
+            if( proc1_pointer == proc2_pointer ) {
+                rc = opal_bitmap_set_bit (bitmap, proc2);
+                if (OPAL_SUCCESS != rc) {
+                    return rc;
+                }
+                ++overlap_count;
+
+                break;
+            }
+        }  /* end proc1 loop */
+    }  /* end proc loop */
+
+    return overlap_count;
+}
+
 static struct ompi_proc_t *ompi_group_dense_lookup_raw (ompi_group_t *group, const int peer_id)
 {
     if (OPAL_UNLIKELY((intptr_t) group->grp_proc_pointers[peer_id] < 0)) {
@@ -71,7 +99,6 @@ int ompi_group_incl_plist(ompi_group_t* group, int n, const int *ranks,
     /* local variables */
     int my_group_rank;
     ompi_group_t *group_pointer, *new_group_pointer;
-    ompi_proc_t *my_proc_pointer;
 
     group_pointer = (ompi_group_t *)group;
 
@@ -99,10 +126,8 @@ int ompi_group_incl_plist(ompi_group_t* group, int n, const int *ranks,
     /* find my rank */
     my_group_rank=group_pointer->grp_my_rank;
     if (MPI_UNDEFINED != my_group_rank) {
-        my_proc_pointer=ompi_group_peer_lookup (group_pointer,my_group_rank);
-        ompi_set_group_rank(new_group_pointer,my_proc_pointer);
-    }
-    else {
+        ompi_set_group_rank(new_group_pointer, ompi_proc_local_proc);
+    } else {
         new_group_pointer->grp_my_rank = MPI_UNDEFINED;
     }
 
@@ -119,50 +144,41 @@ int ompi_group_union (ompi_group_t* group1, ompi_group_t* group2,
                       ompi_group_t **new_group)
 {
     /* local variables */
-    int new_group_size, my_group_rank, cnt;
+    int new_group_size, cnt, rc, overlap_count;
     ompi_group_t *new_group_pointer;
-    ompi_proc_t *proc1_pointer, *proc2_pointer, *my_proc_pointer = NULL;
+    ompi_proc_t *proc2_pointer;
+    opal_bitmap_t bitmap;
 
     /*
      * form union
      */
 
     /* get new group size */
-    new_group_size = group1->grp_proc_count;
+    OBJ_CONSTRUCT(&bitmap, opal_bitmap_t);
+    rc = opal_bitmap_init (&bitmap, 32);
+    if (OPAL_SUCCESS != rc) {
+        return rc;
+    }
 
     /* check group2 elements to see if they need to be included in the list */
-    for (int proc2 = 0; proc2 < group2->grp_proc_count; ++proc2) {
-        bool found_in_group = false;
-
-        proc2_pointer = ompi_group_get_proc_ptr_raw (group2, proc2);
-
-        /* check to see if this proc2 is alread in the group */
-        for (int proc1 = 0; proc1 < group1->grp_proc_count; ++proc1) {
-            proc1_pointer = ompi_group_get_proc_ptr_raw (group1, proc1);
-
-            if (proc1_pointer == proc2_pointer) {
-                /* proc2 is in group1 - don't double count */
-                found_in_group = true;
-                break;
-            }
-        }                       /* end proc1 loop */
-
-        if (found_in_group) {
-            continue;
-        }
-
-        new_group_size++;
-    }                           /* end proc loop */
+    overlap_count = ompi_group_dense_overlap (group1, group2, &bitmap);
+    if (0 > overlap_count) {
+        OBJ_DESTRUCT(&bitmap);
+        return overlap_count;
+    }
 
+    new_group_size = group1->grp_proc_count + group2->grp_proc_count - overlap_count;
     if ( 0 == new_group_size ) {
         *new_group = MPI_GROUP_EMPTY;
         OBJ_RETAIN(MPI_GROUP_EMPTY);
+        OBJ_DESTRUCT(&bitmap);
         return MPI_SUCCESS;
     }
 
     /* get new group struct */
     new_group_pointer = ompi_group_allocate(new_group_size);
     if (NULL == new_group_pointer) {
+        OBJ_DESTRUCT(&bitmap);
         return MPI_ERR_GROUP;
     }
 
@@ -177,52 +193,28 @@ int ompi_group_union (ompi_group_t* group1, ompi_group_t* group2,
 
     /* check group2 elements to see if they need to be included in the list */
     for (int proc2 = 0; proc2 < group2->grp_proc_count; ++proc2) {
-        bool found_in_group = false;
-
-        proc2_pointer = ompi_group_get_proc_ptr_raw (group2, proc2);
-
-        /* check to see if this proc2 is alread in the group */
-        for (int proc1 = 0; proc1 < group1->grp_proc_count; ++proc1) {
-            proc1_pointer = ompi_group_get_proc_ptr_raw (group1, proc1);
-
-            if (proc1_pointer == proc2_pointer) {
-                /* proc2 is in group1 - don't double count */
-                found_in_group = true;
-                break;
-            }
-        }                       /* end proc1 loop */
-
-        if (found_in_group) {
+        if (opal_bitmap_is_set_bit (&bitmap, proc2)) {
             continue;
         }
 
+        proc2_pointer = ompi_group_get_proc_ptr_raw (group2, proc2);
         new_group_pointer->grp_proc_pointers[cnt++] = proc2_pointer;
     }                           /* end proc loop */
 
+    OBJ_DESTRUCT(&bitmap);
+
     /* increment proc reference counters */
     ompi_group_increment_proc_count(new_group_pointer);
 
     /* find my rank */
-    my_group_rank = group1->grp_my_rank;
-    if (MPI_UNDEFINED == my_group_rank) {
-        my_group_rank = group2->grp_my_rank;
-        if ( MPI_UNDEFINED != my_group_rank) {
-            my_proc_pointer = ompi_group_peer_lookup(group2,my_group_rank);
-        }
+    if (MPI_UNDEFINED != group1->grp_my_rank || MPI_UNDEFINED != group2->grp_my_rank) {
+        ompi_set_group_rank(new_group_pointer, ompi_proc_local_proc);
     } else {
-        my_proc_pointer = ompi_group_peer_lookup(group1,my_group_rank);
-    }
-
-    if ( MPI_UNDEFINED == my_group_rank ) {
         new_group_pointer->grp_my_rank = MPI_UNDEFINED;
     }
-    else {
-        ompi_set_group_rank(new_group_pointer, my_proc_pointer);
-    }
 
     *new_group = (MPI_Group) new_group_pointer;
 
-
     return OMPI_SUCCESS;
 }
 
@@ -234,85 +226,65 @@ int ompi_group_difference(ompi_group_t* group1, ompi_group_t* group2,
                           ompi_group_t **new_group) {
 
     /* local varibles */
-    int new_group_size, my_group_rank;
+    int new_group_size, overlap_count, rc;
     ompi_group_t *new_group_pointer;
-    ompi_proc_t *proc1_pointer, *proc2_pointer, *my_proc_pointer = NULL;
+    ompi_proc_t *proc1_pointer;
+    opal_bitmap_t bitmap;
 
     /*
      * form union
      */
 
     /* get new group size */
-    new_group_size = group1->grp_proc_count;
+    OBJ_CONSTRUCT(&bitmap, opal_bitmap_t);
+    rc = opal_bitmap_init (&bitmap, 32);
+    if (OPAL_SUCCESS != rc) {
+        return rc;
+    }
 
-    /* loop over group1 members */
-    for (int proc1 = 0 ; proc1 < group1->grp_proc_count ; ++proc1) {
-        proc1_pointer = ompi_group_get_proc_ptr_raw (group1, proc1);
-
-        /* check to see if this proc is in group2 */
-        for (int proc2 = 0 ; proc2 < group2->grp_proc_count ; ++proc2) {
-            proc2_pointer = ompi_group_get_proc_ptr_raw (group2, proc2);
-            if( proc1_pointer == proc2_pointer ) {
-                --new_group_size;
-                break;
-            }
-        }  /* end proc1 loop */
-    }  /* end proc loop */
+    /* check group2 elements to see if they need to be included in the list */
+    overlap_count = ompi_group_dense_overlap (group2, group1, &bitmap);
+    if (0 > overlap_count) {
+        OBJ_DESTRUCT(&bitmap);
+        return overlap_count;
+    }
 
+    new_group_size = group1->grp_proc_count - overlap_count;
     if ( 0 == new_group_size ) {
         *new_group = MPI_GROUP_EMPTY;
         OBJ_RETAIN(MPI_GROUP_EMPTY);
+        OBJ_DESTRUCT(&bitmap);
         return MPI_SUCCESS;
     }
 
     /* allocate a new ompi_group_t structure */
-    new_group_pointer=ompi_group_allocate(new_group_size);
+    new_group_pointer = ompi_group_allocate(new_group_size);
     if( NULL == new_group_pointer ) {
+        OBJ_DESTRUCT(&bitmap);
         return MPI_ERR_GROUP;
     }
 
     /* fill in group list */
     /* loop over group1 members */
     for (int proc1 = 0, cnt = 0 ; proc1 < group1->grp_proc_count ; ++proc1) {
-        bool found_in_group2 = false;
-
-        proc1_pointer = ompi_group_get_proc_ptr_raw (group1, proc1);
-        /* check to see if this proc is in group2 */
-        for (int proc2 = 0 ; proc2 < group2->grp_proc_count ; ++proc2) {
-            proc2_pointer = ompi_group_get_proc_ptr_raw (group2, proc2);
-            if( proc1_pointer == proc2_pointer ) {
-                found_in_group2 = true;
-                break;
-            }
-        }  /* end proc1 loop */
-
-        if (found_in_group2) {
+        if (opal_bitmap_is_set_bit (&bitmap, proc1)) {
             continue;
         }
 
+        proc1_pointer = ompi_group_get_proc_ptr_raw (group1, proc1);
         new_group_pointer->grp_proc_pointers[cnt++] = proc1_pointer;
     }  /* end proc loop */
 
+    OBJ_DESTRUCT(&bitmap);
+
     /* increment proc reference counters */
     ompi_group_increment_proc_count(new_group_pointer);
 
     /* find my rank */
-    my_group_rank=group1->grp_my_rank;
-    if ( MPI_UNDEFINED != my_group_rank ) {
-        my_proc_pointer = ompi_group_peer_lookup(group1,my_group_rank);
-    }
-    else {
-        my_group_rank=group2->grp_my_rank;
-        if ( MPI_UNDEFINED != my_group_rank ) {
-            my_proc_pointer = ompi_group_peer_lookup(group2,my_group_rank);
-        }
-    }
-
-    if ( MPI_UNDEFINED == my_group_rank ) {
+    if (MPI_UNDEFINED == group1->grp_my_rank || MPI_UNDEFINED != group2->grp_my_rank) {
         new_group_pointer->grp_my_rank = MPI_UNDEFINED;
-    }
-    else {
-        ompi_set_group_rank(new_group_pointer,my_proc_pointer);
+    } else {
+        ompi_set_group_rank(new_group_pointer, ompi_proc_local_proc);
     }
 
     *new_group = (MPI_Group)new_group_pointer;