For multiple people, posting is basically a random free-for-all after the GM makes their first post unless the GM imposes some kind of a rule about posting order. In terms of how to actually RP, you pick another player to interact with, and then involve people who join the interaction or do something that would affect it. 

EDIT: An example of an interaction would be:

Bob talks to Alice
Steve can butt in, so he does
Bob, Steve, and Alice are now talking to each other.

...Something like that. It isn't too hard to grasp once you get into it, really.