I should really build some type of club house/meeting spot I guess, but I usually use it for friendly get-togethers at a sims home.
One of my families is a married couple and the wife has the family-oriented trait, so she has a club that is simply called "Family" and once or twice a week she will start a "meeting" and her children that are grown and no longer live in the same house will show up with their spouses. For the most part they just sit around talking, watching TV, and generally just being nice to each other (those are basically all the rules). Eventually her husband will cook a huge meal and they all sit down to eat.
Another one is my nerdy character who has his friends in the "Friend" club. Once a week or more they show up at his house and play games until the early hours of the next day. Kind of like my old D&D days with my friends before we all grew up and had responsibilities.
Other than that I've had a few exercise clubs for my athlete type characters. Usually it's just going to the gym with them, except for my one rich sim who has enough equipment at his house for everyone to use at once.
I tried making a not so nice biker type club once. The main rules were pretty much to be mean and fight everyone not in the club. But sims are too nice :D They might (really just a might), say one mean thing to another sim then start talking to them like nothing happened.
