i don't think Michael was the leader of the crew, i doubt they even had a leader...
That's exactly why though. Michael was the head of the crew. Michael was doing sh*t like this before he met Trevor. Him and Trevor may be best friends, but Trevor worked under Michael. Trevor was part of Michael's crew.
Michael put together the scores with the help of Lester - who was more of a "behind the scenes" kind of guy. Hell, even in the story Trevor says a few times with variations of "my score" - because all the others were Michael's scores. They were set up by Michael, and when going into them Michael was the head of the group.
It astounds me how many people here are trying to come up with another explanation, when the answer is right there in the game. You're all making it more complicated than it needs to be.