I had the first three players meet at a tavern after being called there separately by a group who had noticed their previous actions (I was starting them above 1st level) and thought they could use their help. The other two characters were introduced a session later, when the person who called the meeting brought them to their meeting place (they were uh, sidetracked in the first adventure). They were introduced as already having been a part of that meeting caller's organization.
In another group, it started off as a solo quest. Then the second PC jumped in to help out in a brawl out in a town common.
In another group, it started off as a solo quest. Then the second PC jumped in to help out in a brawl out in a town common.