I noticed that not all the players are listed in the chat room. How do you opt out?
Don't login to lichess. Then when you visit the arena, your username will not show in the chat
If I don't log in, I can't play. There are often 50 people playing (so they have to be logged in), but maybe only 12 listed in chat?
I think what you mean is not the people in the chat, but in the tournament "lobby". I think when you are playing, you are not in that list.