Tom,
I've been to cozumel twice,(40 dives), and I consider drift diving there to be" easy chair" diving-get neutral, relax,and go with the flow.
My experience has been : each small group enters together, descends with its' DM, and enjoys the ride. At the time limit, one has been set , the group surfaces. Alternatively, if a buddy team needs to surface earlier, the do so and are picked up by the boat, which has been following "bubble watching"-at a safe distance.
The pick up is something to see- you are in the water watching as the boat captain backs towards you-with pinpoint precision- interesting rather than scary.
While each shop/boat probably has its own variation on this process, most that I saw followed a similar drill.
Hope you enjoy your trip ,
Miked