Why might he need companions? Because he was entering mid to end-game level territory.
In the novel, William was nearly level 50 when he 'stumbled' upon the Hermit somehow—though he tore through all of his opposition with ease, Mark knew the Mutant Realm wasn't to be taken lightly.
Having powerful allies would help his quest.
He himself was barely level 30, so the Mutant Realm would pose a significant challenge. Stats differences weren't the end-all-be-all—skill; trump cards would often triumph in real battle.
That's where companions came into play. Humans were social creatures by nature. Survival and success was found in numbers. But that itself was no easy endeavor—at least for Mark.
He had done a shabby job of friend-making, especially with the whole 'antagonize the entire upper class' bit that he had performed at the duel. Though within Class 1-A he had made a few acquaintances, there wasn't anything too serious.