How do I bring characters together at 1st level? Culturally, IMC, many people are organized into 'parties'. At around seven years old, children are generally gathered into groups of friends that are expected to last for a lifetime. For the lower classes, who do this less often, it may just be the local kids who are together. For nobles, prospective children of other nobles and important people are looked over and then they come to an agreement and all the children typically go to live in one estate. These groups are expected to compiment eachother and form lasting freindships. Thus, the king's son may end up in a "party" with the high clerics son, the court wizard's daughter, and maybe even the son of a rich nobleman (who also happenes to be well placed in the theives guild). The theory being that they will grow up together and support eachother in later life. Although they go to separate career training at the age of 14 or so, when they are done, they are brought back together and expected to support eachother throughout their lives. "Adventuring" is a somewhat respectable career especially for the extra sons and daughters of nobles and rich persons who can't be expected to inherit their parant's postion or wealth. Instead, they are prepared for life by their parents by being provided with such a 'party'. Thus the 6th son of a nobleman can't expect to inherit any lands or even a decent marriage, so the father may go so far as to pick out gifted commoner's children to have raised with his son and then later train them in various classes with the expectation that they will allow his son a chance as a mercenary or adventurer in a group he can trust to support him. Although this is usually a tactic used by the noble or well to do, it is often mimiced by groups of lower class children who grow up together and gives a reason why they all know eachother.
Other than that, I let them come up with their own reasons for knowing eachother or come up with a way drawing upon details of their characters backgrounds. In the end, it is sort of handwaved and the odd character is enlisted to be part of the adventuring party and various role playing that allows them to be trusted over the rest of the unwashed masses has been assumed to already been played out.