It really seems to depend on the people who are roleplaying. Some are ok with others jumping into their roleplay and others aren't. Personally I'm OK with a random person just joining in (as long as it fits the mood) to anything I'm roleplaying in public at the time. If I wanted something people couldn't bump into, well, I'd roleplay in private. As someone who has encountered a TON of cliques though (even amongst friends), I will usually send a private message first to see if it's ok to jump in.
