Index: bp-groups/bp-groups-loader.php
===================================================================
--- bp-groups/bp-groups-loader.php
+++ bp-groups/bp-groups-loader.php
@@ -215,7 +215,7 @@ class BP_Groups_Component extends BP_Component {
 		) );
 
 		// Auto join group when non group member performs group activity
-		$this->auto_join = defined( 'BP_DISABLE_AUTO_GROUP_JOIN' );
+		$this->auto_join = defined( 'BP_DISABLE_AUTO_GROUP_JOIN' ) ? false : true;
 	}
 
 	/**
Index: bp-groups/bp-groups-screens.php
===================================================================
--- bp-groups/bp-groups-screens.php
+++ bp-groups/bp-groups-screens.php
@@ -140,7 +140,7 @@ function groups_screen_group_forum() {
 				check_admin_referer( 'bp_forums_new_reply' );
 
 				// Auto join this user if they are not yet a member of this group
-				if ( $bp->groups->auto_join && !is_super_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) )
+				if ( bp_groups_auto_join() && !is_super_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) )
 					groups_join_group( $bp->groups->current_group->id, $bp->loggedin_user->id );
 
 				$topic_page = isset( $_GET['topic_page'] ) ? $_GET['topic_page'] : false;
@@ -337,7 +337,7 @@ function groups_screen_group_forum() {
 				if ( $user_is_banned ) {
 				 	$error_message = __( "You have been banned from this group.", 'buddypress' );
 
-				} elseif ( $bp->groups->auto_join && !is_super_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) ) {
+				} elseif ( bp_groups_auto_join() && !is_super_admin() && 'public' == $bp->groups->current_group->status && !groups_is_user_member( $bp->loggedin_user->id, $bp->groups->current_group->id ) ) {
 					// Auto join this user if they are not yet a member of this group
 					groups_join_group( $bp->groups->current_group->id, $bp->loggedin_user->id );
 				}