Lots of responses, so I'm late to the party, but I think this is one of those times when there is no hard and fast etiquette rule. Every group has its own dynamic and I think the etiquette requirement is to behave within that dynamic. Which, if you (the "general you") have been a member of the group for a while, you will know.
To give an example from my life: DH and I belong to a running group that always meets on a certain night of the week, and following the run, the group always goes to the same pub. The number of people who might choose to show up for the run is huge, but on any given evening, only a subset is there. And we don't all run together, but divide ourselves into groups of people running a similar pace. Anyone is welcome to bring a new person (i.e.: a guest), and as long as that person can run the pace of whatever group he/she joins up with, it's all good. Anyone who brings a new person will take on the responsibility of making sure that person is matched with the right group. It sounds confusing but in practice it all works out.
And just like the running part, anyone is welcome to go to the pub (we have a standing reservation that can be expanded or contracted quite easily; the place is never all that busy on a weekday evening), but only a smaller subset shows up there consistently. In practice, most people coming to the pub for the first time are accompanying a friend who is usually there. The group at the pub is "tighter" than the larger running group, but new people are always welcome. It's not necessary to clear it with other group members first. Presumably if you were to bring someone who simply didn't mesh, somebody might suggest that you not bring that person again, but in practice I haven't seen this happen.
But where it changes is when the "pub group" is planning other social activities. Those tend to be invitation only and if I wanted to bring someone else, I would definitely bounce it off a few people first. Every year a small group of us have a beach party on a public-but-not-easily-accessed beach, which usually goes well into the early morning, and is most definitely a "closed" party for people who know each other well. It is unlikely that anyone would be invited who had not already become a regular member of the pub group, and it is one of the few situations where I (who am typically a "more the merrier" person) would be annoyed to have a relative stranger or two show up.
So, in summary, IMHO, the only true etiquette issue is to be sensitive to a given group's way of operating.