There are none. It's the Texas Cage Match of road riding.
I took the previous sentence and modified it from the "rules" of the Breckenridge Epic mountain bike stage race. Their Rule #1 made me laugh and made my Twitter page.
Kidding aside, while there are no "rules" for the Sunday group ride, there are a few notes of good manners that have come up repeatedly over the past six months or so that have been begging for comment. Our riders tend to comment to each other, and me, while other groups would chastise bad behavior by screaming at the offender on the spot, or delivering some sort of punishment on the bike or off the bike (tar and feathers).
Anyone that shows up for the ride is responsible for other people on that ride. If every person on the ride looks out for the person on their wheel, then everyone is covered. Knowing only what is going on in front of you, with no concern for what is behind you, is best left for your solo effort on race day.
At the beginning of the ride I usually ask who is going the entire distance of the planned ride. Speak up. If you are not planning on going the entire ride distance, let everyone know. Then, if you decide to launch an attack off the front or on a hill, people riding the entire distance can decide if they want to let you hang out there or chase you down. Stirring up the group and then turning around early, without forewarning, is a show of bad manners (BM).
If the group stops for food or a pit stop and you decide to keep rolling, let someone(s) know. That person(s) should let others that are stopped know as well. If you keep rolling, it is generally assumed you are riding easy until the group joins you or rolls past. For most everyone, there are days when the group is riding faster than you can muster so it is fine to keep rolling to ride by yourself at the front rather than riding by yourself at the back, particularly if you are going longer than others. After informing others you are going to keep rolling, be sure they know if you intend to keep on the planned route or you are going to turn off somewhere. Of course, riding through the stop and then claiming the next city limit prime, hill bonus or time trialing is BM.
When the group rolls out of a stop, take a moment to look around for bikes without riders. It is no fun to come out of a restroom to find the group has left without you.
If your legs are not cooperating and you don't want the group to wait for you, let someone know because generally, people are doing bullet #1.
If you have announced you are having a bad day, tired legs, etc. it is fine to sit in for the entire ride. This is a nice way that the strong people can get a workout and you can as well. Of course, we all know it is a fine showing of BM (and not superior fitness) to feign weary legs, sit in, and then attack when everyone else has been doing the work.
Finally, because we get new people dropping into the group all the time, the entire group is counting on you to help shape good manners on the ride. Most of the time, your good behavior or a few helpful words are enough to help people understand how the group works. No one wants the label of being the group's BM.