When removing user from a group, only reassign site role when necessary.

* Don't do it if 'silent_add' is disabled.
* Don't do it if the user is not already a site member.

bp-groupblog.php

@@ -450,11 +450,22 @@ function bp_groupblog_changed_status_group( $user_id, $group_id ) {
  * Called when user leaves.
  */
 function bp_groupblog_remove_user( $group_id, $user_id = false ) {
+	// Only modify site membership if the plugin is configured to do so.
+	$settings = bp_groupblog_get_group_settings( $group_id );
+	if ( ! $settings['groupblog_silent_add'] ) {
+		return;
+	}
+
 	$blog_id = get_groupblog_blog_id( $group_id );
 
 	if ( ! $user_id )
 		$user_id = bp_loggedin_user_id();
 
+	// Users with no existing role should not be modified.
+	if ( ! is_user_member_of_blog( $user_id, $blog_id ) ) {
+		return;
+	}
+
 	$user = new WP_User( $user_id );
 	$user->for_blog( $blog_id );
 	$user->set_role( 'subscriber' );