When going to events like play parties, it can happen that (after negotiation) other Doms or subs are invited to participate in a play scene.
While it might seem fun to join in, never join into any scene that's already in-progress unless you have explicit permission. You should never interrupt a scene to get that permission. Stick to watching the scenes unless you've negotiated joining in before the scene starts. That said, if you watch a super-hot scene between people and now would like to play with someone from the scene, wait until the scene is over, give them some time to wind down, and then approach and ask about the possibility of playing with that person. Remember to always be nice if the answer is no.