I do that a lot. xD It's pretty common for somebody to ignore me whilst I do this, but most likely they're not interested or simply don't have the time or don't even want to RP with somebody else besides the people he/she is currently engaged in RP with. However, we can always ask to hop in, I don't see a problem with it, and I don't see it necessarily rude either. It's most likely that people are too 'in' the RP to care about anybody else. But that's just my opinion. ^^