Multiple user profiles like in seblod?

I am not exactly sure what t is. It sounds like seblod has it is own user registration form. Or can add additional fields to joomla core user registration from.

In any case Cobalt do not o anything of this. Cobalt neither extends user registration or has its own user registration. Thus no user profiling system.

But we do plan tightly integrate Cobalt with other profiling systems like jomsocial, CB, ..

Just an idea: Why not register with Joomla core or any other user extension... then use a special Cobalt section for "extended user-profiles". Remember, you can limit the entries to 1! ;-)

Is it possible to show the J!usergroup selectbox at all?

Yes it is possible.