bp_get_group_cover_url( object|bool $group = false )

function bp_get_group_cover_url( $group = false ) {
	$group_id = bp_get_group_id( $group );

	if ( ! $group_id ) {
		return '';
	}

	$cover_url = bp_attachments_get_attachment(
		'url',
		array(
			'object_dir' => 'groups',
			'item_id'    => $group_id,
		)
	);

	if ( ! $cover_url ) {
		return '';
	}

	return $cover_url;
}