Read the manual, read the notations about permits in the RMP and then give the BLM office a call. A little due diligence on your part will go a long ways. They can give you the final answer.
My quickee notes:
If your going to be charging for your event, you need a permit.
If your going to be advertising your event, you might need a permit.
If your event is hosted by a commercial or organized (club) entity, you might need a permit.
If you need a permit, you need to apply at least 180 days in advance.
If you need a permit be prepared to pay for the BLM's time to prep the permit
I've got a more 'compiled' list that came from the SLC BLM district that was seemingly far more restrictive, I'll see if I can find it.
As I have ranted in the past, these permit processes could really change the face of organized OHV runs right down to a family reunion or monthly club run