To actually get characters together, I'll tell about another campaign I was running:
The original party went into the Temple of the Snake Demons (yuan-ti), where a couple of the characters died while cleaning the place out. They limped back to town to recover. While in town a couple of the slaves they had freed asked to join their rescuers. Dave was really unhappy playing a ranger, so I allowed him to restart with a mage. The ranger was found face down in an alley, obviously robbed. Seamus (I forget his character's name) was a thief who wanted to get in some practice, so he went to rob a house - incidentally, the house of Therat (Dave). He was captured, apparently blood-cursed, and released. He went back to his inn, where Ramirez (6'4" Moorish Blade (bard)) was sitting in the common room and thinking about joining a game of dice. This fellow coming through the door with a bloody and a rune drawn on his forehead in blood looked interesting, so Ramirez followed him and tried to talk his way into their (Seamus and Mike (warpriest)) room. He failed and was walking back to his table when the innkeeper passed him in the hall - obviously going to check on his guest. The different voice saying he was the innkeeper got Mike's attention - there was obviously something wrong. In order to protect his interests, Ramirez threw a dagger and killed the innkeeper.
Other events occurred that involved lots of running around, getting picked up by the guard (Seamus lost three sets of lockpicks to the guards in two days :), and being 'encouraged' by the Thieves Guild to leave town the party was high-tailing it for the hills - not to avoid the guard but to escape the guild - Ramirez killed one of the Guild representatives escorting Seamus out of town.
In short, arrange things so that the party (even the innocent members) end up fleeing the city with the law/guild behind them. They will be forced to work together in order to stay alive...