The 2 mile two track out to the trailhead is used as a warm up, but there is also a 1/2 mile piece of singletrack involved in that. Generally in mountainbike group rides everybody tends to spread out and regroup so nobody gets lost or left behind too far.
At any intersections the riders regroup so everybody gets to ride a pace that works for them. Especially in large groups, with a large difference in rider abilitiy, the slower riders will feel that they may have been left but just need to understand that you ride your ride and the guides will regroup where it's not clear which direction to go.
Also we try to all ride ride the same direction, (KEN!), so if someone finds themselves in a faster group then they can drop back to the next group. Always talk to a ride guide before the start to make sure you get in the group that you need to be in.