Yep, just post in any active RP anytime you wish. Just make sure you read the title of it first, noting that certain restrictions are not placed on it. i.e. High Stakes ((Exclusive RP, DM if not the List))
Others will say something like "Invitation Only" or "DM before Posting", stuff along those lines. Obviously, there is no way to stop someone from posting in one of these "restricted" RPs, but generally there is a level of respect associated with these and others don't tend to intrude upon them. Generally speaking, these "Invitation/On the List/DM before Posting" threads are headed by either a more experienced RPer, or one that is simply attempting to keep newer RPers, that may be less experienced, out. Most of 'em are pretty considerate and will allow you to come on in as soon as they've seen some of your work, albeit it is actually good.
I talk with a decent amount of experience on that, being a more active RPer that is actually heading an RP with the title above. Hope to see you start RPing in the Meeting Hall, more RPers are always welcome as far as I'm considered!