Honestly? Steal the one really good reason for sticking together Heroes produced (and yeah, the writing went haywire with Heroes - seems Kring didn't know where to go): Serial Hero-Eater. Make them targets. Fear and danger drives people to either self-implode or co-operate. It's natural.
Introduce a Sylar, one which likes a challenge. Have him devour (and I mean devour, not the fiddly crap Gabriel Gray does) a bunch of heroes and it hitting the news. Make sure his Modus Operandi is well-known (terrorizing his victims?) and start hitting the players up with "pre-meal" tactics. Perhaps your "Sylar" is tired of eating single goons and to "up his ante" he is driving them together in the hopes of producing a true challenge.
For more fun. Make him a Professor at Emory University, thus giving him knowledge of who has these potential powers. Want to make it even MORE fun? Have him appear as a mentor, someone who wants to help them as part of his research. That way, your Sylar manipulates the entire scenario for his own benefits, while giving the group reason to work together and get to know eachother.