Hi, and welcome to Ulysses.
Yes, it's been asked before, but, to be quite honest, it actually took me ~5-10 minutes to find the answer.
Even though it's been asked/answered, it's not that clear to find unless you already know what to look for.
You'll want to see this thread, somewhat mis-titled with ALL the functionality it covers, not just immunity.
Immunity: How It Works NowRead the first and second posts, they'll show some clarity of the ulx command line parameters for adduser/groupallow that should assist you greatly.
Megiddo gives specific examples of how to allow certain groups temporary ban times and allowing of banning to only certain equal/lesser groups.
As the original post author suggests, backup your ulx/ulib data files in case you mix up any.
Stickly man would poke me for not stating, the commands can be done with XGUI too.