BuddyPress.org

or:

function bpex_user_can_create_groups( $can_create, $restricted ) {

	$groups = groups_get_groups();

	if ( $groups['total'] <= 70 )
		return $can_create;

	return false;
}
add_filter( 'bp_user_can_create_groups', 'bpex_user_can_create_groups', 99, 2 );