According to Per Jorner's Fallout 2 Guide, which I've been referring to (not using it for everything of course, just when I'm really stuck), says that after you do quests 1. and 2. in Broken Hills "Find the Missing People" and "Fix the mine Purifier", Marcus will join you.
But to me he still just says there are too many people and he won't join a crowd anymore.
My Karma is 799 and I'm not a Slaver, Childkiller, or a mutant hater or anything.
Honestly, if I have to get rid of Sulik or Vic I probably would because Marcus is really cool. But, I can't figure out how to fire anyone.