It isn't clearly mentioned in the show, not I have read the original content. Through my little running mind, I have speculated some possibilities:

1) Follow the Leader: The leader is the FL as she first started experiencing the loop. Maybe if only she drags out someone out of the bus like she did with the ML, then you can enter the loop. This a physical touch theory, means she explicitly has to touch the other person.

2) Follow the Lead: Maybe you just have to get off the bus, no FL touch or even FL ordering is needed. But getting off the bus has to be after the bomb enters the bus. This properly explains why Lu Di couldn't enter the loop .

3) Sleeping is the Entry: The only two people who were able to enter the loop was sleeping in the bus, but no one else was sleeping soundly. Though it doesn't explain the ML's late entry to loop.

4) Not the choosen one: Yes, it's as simple as that. Maybe it's the spirit of the deceased girl who chooses who will be in the loop.