If you need to add your new group to a group set you could get by ID or find it by name

$yourGroupSet = GroupSet::getByID($gsID);

If you want to create a group set on the fly

if(!is_object($yourGroupSet)) $yourGroupSet = GroupSet::add('Group Set Handle', $pkg);

You should then check if group exists and create

if(!is_object(Group::getByName('New Group Name'))) {
        $g = Group::add('New Group Name', 'New Group description');
        $yourGroupSet->addGroup($g);
}

Once the group is added you can then add users to it

$u = new User(); //get the current user
//check the user is logged in as you cant assign groups to people who dont have an account
if($u->isLoggedIn()){
        //if user not already in new group
        if(!$u->inGroup(Group::getByName('New Group Name'))){
                $u->enterGroup('New Group Name');
        }
}

You can also remove people from groups with

$u->exitGroup('New Group Name');

You could (and probably not really recommended but we've had to do it before) add non-logged in users to a group temporarily for that session... Get in touch if you need a hand with that - its not pretty!