I've just looked in FAQ and found:
Yes! Go to the Events tab and, as a
member, add event. Non commercial, community events only please.
Events advertising commercial ventures and retail events like sales
will be removed."
I think Clive has it right, people will post an "expression of interest" type of post in general discussion to see if anyone else is available and where they would like to go. If interest is big enough it usually ends up as an event.
I'm riding to OH today with a mate and mentioned it in a discussion and I think have two more riders. i didn't want to create an event, as such, because I just want a quiet ride and it was a last minute decision.