I belong to the Orlando Jacks club (nearest one to me), and have attended others around the country. The rules are all fairly similar to these from the Orlando club, although some do permit private play, away from the group scene, where anything goes.
What is allowed:
Sensual touching of one another
Jacking off of one another
Nipple play
Foot play
Cum play
Kissing
Voyeurism
Exhibitionism
Massaging
Erotic talk
Gay Pornography
Lube (Astro Glide, Albolene, Boy Butter, etc.)
Anonymity (must provide only us with basic info)
Only watching, but you must be naked
What is NOT allowed:
No fucking
No sucking
No lips below the hips
No aggressive behavior
No forcing anyone to do something with which they are not comfortable
No playing in any rooms except the designated "play room"
No cameras of any type
No audio recording devices of any type
No swapping of play toys
No sharing of bodily fluids (in one another's orifices)
No disclosing any personal information about attendees or the location of the event
No alcohol is allowed
This is for a club at a private residence, and the hosts (the homeowners) get to set the rules.
As far as unorganized jacking parties (like you'd arrange at a hotel with Craigslist), the rules generally are less stringent. Basically, no means no, but you can give anything a try once. Like every other situation in life, just ask. "Mind if I suck your cock?" "Do you want to cum on my face?" "Need a hand with that?" And so forth.