There is no one set of definitive rules when it comes to etiquette on Mu*s. It's different on different games and in different genres. So it's easy to carry over expectations from another game on what is considered polite. This post is to let everyone know what the staff consider polite, based on our past history on various games and therefore what we consider to be polite here.
1) Even if a scene is in a public room, it is polite to page and ask if they would mind another person. You could page everyone or just one person, expecting they'll ask the others. In either case, paging is polite.
1a) If you get told no, realize that different people have different limits on how big a scene they can handle. Even if it's just 2 people, they could be short on time and adding an extra person will mean they won't get to finish the scene.
2) If a scene is in public, realize that some people come from games where just walking in is acceptable. Don't assume they're being deliberately impolite. If you'd prefer they not join, politely let them know. A reason why is optional.
2a) Also, playing in a +hangouts jump point or an entrance is going to get people walking through.
3) If a scene is private, don't play in a public room. That's just common sense. We have RP Rooms and Temproom code.
Please note that staff is not going to police etiquette. According to the rules here, we're all adults. You can choose to be polite or not whether you're the one wanting to join a scene or having someone want to join. Just be aware that not being polite has social consequences here as in any community.
That being said, we will definitely come down hard on harassment, stalking someone when asked to leave them alone, etc. If someone is doing that, tell us. If you're not sure if it qualifies, tell us anyway. If someone tells you to leave them alone, leave them alone. We'd really prefer not to ban anyone but anyone who can't play well with others is not welcome here.