Index: bp-themes/bp-default/groups/single/forum/topic.php
===================================================================
--- bp-themes/bp-default/groups/single/forum/topic.php	(revision 3138)
+++ bp-themes/bp-default/groups/single/forum/topic.php	(working copy)
@@ -83,7 +83,7 @@
 					<div id="post-topic-reply">
 						<p id="post-reply"></p>
 
-						<?php if ( !bp_group_is_member() ) : ?>
+						<?php if ( !bp_group_is_member() && !BP_DISABLE_AUTO_GROUP_JOIN ) : ?>
 							<p><?php _e( 'You will auto join this group when you reply to this topic.', 'buddypress' ) ?></p>
 						<?php endif; ?>
 
Index: bp-themes/bp-default/groups/single/forum.php
===================================================================
--- bp-themes/bp-default/groups/single/forum.php	(revision 3138)
+++ bp-themes/bp-default/groups/single/forum.php	(working copy)
@@ -25,7 +25,7 @@
 
 				<?php do_action( 'bp_before_group_forum_post_new' ) ?>
 
-				<?php if ( !bp_group_is_member() ) : ?>
+				<?php if ( !bp_group_is_member() && !BP_DISABLE_AUTO_GROUP_JOIN ) : ?>
 					<p><?php _e( 'You will auto join this group when you start a new topic.', 'buddypress' ) ?></p>
 				<?php endif; ?>
 
Index: bp-groups.php
===================================================================
--- bp-groups.php	(revision 3138)
+++ bp-groups.php	(working copy)
@@ -6,6 +6,9 @@
 if ( !defined( 'BP_GROUPS_SLUG' ) )
 	define ( 'BP_GROUPS_SLUG', 'groups' );
 
+if ( !defined( 'BP_DISABLE_AUTO_GROUP_JOIN' ) )
+	define ( 'BP_DISABLE_AUTO_GROUP_JOIN', false );
+
 require ( BP_PLUGIN_DIR . '/bp-groups/bp-groups-classes.php' );
 require ( BP_PLUGIN_DIR . '/bp-groups/bp-groups-templatetags.php' );
 require ( BP_PLUGIN_DIR . '/bp-groups/bp-groups-widgets.php' );
@@ -364,7 +367,7 @@
 				check_admin_referer( 'bp_forums_new_reply' );
 
 				/* Auto join this user if they are not yet a member of this group */
-				if ( !is_site_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) )
+				if ( !is_site_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) && !BP_DISABLE_AUTO_GROUP_JOIN )
 					groups_join_group( $bp->groups->current_group->id, $bp->loggedin_user->id );
 
 				if ( !$post_id = groups_new_group_forum_post( $_POST['reply_text'], $topic_id, $_GET['topic_page'] ) )
@@ -542,7 +545,7 @@
 				check_admin_referer( 'bp_forums_new_topic' );
 
 				/* Auto join this user if they are not yet a member of this group */
-				if ( !is_site_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) )
+				if ( !is_site_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) && !BP_DISABLE_AUTO_GROUP_JOIN )
 					groups_join_group( $bp->groups->current_group->id, $bp->loggedin_user->id );
 
 				if ( !$topic = groups_new_group_forum_topic( $_POST['topic_title'], $_POST['topic_text'], $_POST['topic_tags'], $forum_id ) )