bp_groups_disable_at_mention_forums_notification_for_non_public_groups( bool $send, array $usernames, int $user_id, int $forum_id )

function bp_groups_disable_at_mention_forums_notification_for_non_public_groups( $send, $usernames, $user_id, $forum_id ) {
	// Skip the check for administrators, who can get notifications from non-public groups.
	if ( bp_user_can( $user_id, 'bp_moderate' ) ) {
		return $send;
	}

	// Get group ID's for this forum
	$group_ids = bbp_get_forum_group_ids( $forum_id );

	// Bail if the post isn't associated with a group
	if ( empty( $group_ids ) ) {
		return $send;
	}

	// @todo Multiple group forums/forum groups
	$group_id = $group_ids[0];

	if ( ! bp_user_can( $user_id, 'groups_access_group', array( 'group_id' => $group_id ) ) ) {
		$send = false;
	}

	return $send;
}