Have you looked at function bp_group_has_members in buddypress\bp-groups\bp-groups-template.php ?
You can't pass a string '$group' to bp_group_has_members.
Try:
$this_id = bp_get_group_id();
echo "group id is: " . $this_id; // make sure there is a value here.
$has_members_str = "group_id=" . $this_id;
if ( bp_group_has_members( $has_members_str ) )