The standard rules of the road apply...dont tailgate, use your signals, dim your hi-beam for oncoming trafic at night, etc. MSF has some great tips for group riding.
One thing that comes to mind is when faster riders come up behind me, I slide over to the berm side of my lane to give them a little extra room to get around. Then there's the whole waving thing too. I try to always wave at other riders but if I miss one I dont feel bad about it. And if I do wave and I dont get a wave back, thats OK too.
The biggie is to respect other riders and their rides. Dont brand/style bash and be patient with unexperienced riders.
And for God's sake, if you see a really cool bike in a parking lot, dont sit on it without the owners permission. Thats a good way to get your ass kicked.