Index: bp-groups/bp-groups-activity.php
===================================================================
--- bp-groups/bp-groups-activity.php	(revision 9404)
+++ bp-groups/bp-groups-activity.php	(working copy)
@@ -345,6 +345,7 @@
 
 	groups_update_groupmeta( $group_id, 'last_activity', bp_core_current_time() );
 }
+add_action( 'groups_join_group',           'groups_update_last_activity' );
 add_action( 'groups_leave_group',          'groups_update_last_activity' );
 add_action( 'groups_created_group',        'groups_update_last_activity' );
 add_action( 'groups_new_forum_topic',      'groups_update_last_activity' );
Index: bp-groups/bp-groups-functions.php
===================================================================
--- bp-groups/bp-groups-functions.php	(revision 9404)
+++ bp-groups/bp-groups-functions.php	(working copy)
@@ -439,9 +439,6 @@
 		'user_id' => $user_id,
 	) );
 
-	// Modify group meta
-	groups_update_groupmeta( $group_id, 'last_activity', bp_core_current_time() );
-
 	do_action( 'groups_join_group', $group_id, $user_id );
 
 	return true;