We get lots of email asking the same basic question in many variations...
The simplest way to fit in in a play party is to behave politely. There will be people right in front of you who are doing very sexual things. They are doing them for _their_ pleasure, not for yours. Stay away from the action unless invited to participate -- and a glance in your direction does not constitute an invitation.
The people who really interfere with the energy of a party are the people who seemingly assume that just because the scene is taking place in a semi-public context means that comments from the audience are okay fine. They're NOT. The top in the scene may be concentrating on the bottom's pleasure, and the bottom is almost certainly in a very private mental space. Neither the top nor the bottom will appreciate being yanked back to reality by a loud suggestion or greeting. If you want to compliment them on something, WAIT UNTIL THE SCENE IS OVER and they're circulating and being sociable again! Interfering with a scene in progress is inexcusably rude, and if I were dungeon-mastering I would throw you out of the party for doing it.
Once you understand that scenes are private even though they're taking place in public, the question then becomes...
There definitely are people who interfere just by watching. They've been dubbed "energy vampires" in the past. These people are watching the action as though it was a porno movie -- as though the intense magic taking place in front of them was no more than a bad fuck flick where the actress is half asleep. They have no empathy, no sense of connection to what's going on; they might as well be in a movie theater.
If you have the ability to watch what is happening with an open heart, if you can pick up on the energy and send your own good wishes towards the participants in the scene, you will be much more valuable as a watcher. Public players never object to an enthusiastic audience which can appreciate the way they're playing! An audience which values the gift of being allowed to watch, and which contributes its goodwill towards the play, can be a delight; an audience which watches without giving and without connecting takes the life and spirit out of the scene. (And remember, a good audience does NOT make comments that the players can hear -- an audience doesn't interfere with the performance!)
You can be a part of the magic without playing yourself. All it takes is an honest enjoyment of what's happening combined with politeness and tact.
If you _do_ want to play, and there's someone you want to play with, you can ask -- but be prepared to accept a "no, thanks" gracefully. If you're comfortable mingling and making small talk, you'll be more likely to find someone with compatible desires -- after all, everyone else there has similar tastes! There often will be rooms for heavy play and rooms for hanging out and socializing; don't try to do one activity in the other activity's space. This is an important distinction: if you want to talk, take it somewhere besides where someone else is trying to create a scene. And in general, don't just start a scene in the midst of other people's conversations.
One last note: it helps if you dress sexily, even if you're not playing -- usually the more leather and lace there is to look at, the better!