Depending on the players, I either give them their group background, ("smiley, the fixer you all know picked you all for a run a few months back. The run went like clockwork, and you all decided to stick together after that.") Or, I let them come up with their own backstory for the group. Seen some really great runs too! (like the elven identical triplets and their Ork "muscle", to all being from the same barrens gang that pulled off a large score right before the rest of the gang got slaughtered by the Corp reprisal team.)
Things get trickier when I have to introduce a new player... Then fall back alot on Fastjack's "fixer drop in" option. Also use the "common allies" trick sometimes as well... Again, it all depends on the players and their style of play.