I can answer that, if the Council doesn't mind. People usually note in the thread title or opening post who is allowed to participate; if they don't, it's probably safe to assume that the people involved know who they are and won't welcome meddlers. So unless you see a thread that specifically notes that anyone is allowed to jump in, or that there are X many open spots, you probably shouldn't.

If you're still unsure or really want in on a particular piece of action, though, you could always PM the people already involved. They might be amenable to an extra person, and the worst they can do is say no.