I had a group at one point where they pretty much all built loner characters. I decided rather than stretching the laws of credulity as to why all these independents suddenly wanted to work as a team I'd take a leaf out of the first episode of the TV show Leverage.
Have them contracted out individually on a job, something that runs on the grey side of the law, and when they come to collect payment, have their employer doublecross them, leaveing them for dead. The obvious mission then becomes to track down their previous employer and enact some form of payback, but because the employer is either high profile (in the mundane or supernatural world); backed by some greater threat; or just plain difficult to track down, the PC's will have to keep working together to do so.
