They are as varied as the people in them.
So what I post here is just very general characterizations.
You will find them informally on yahoo and Meet-up. I know one in my area w. some highly skilled seakayakers (a good number of them are ACA or BCU instructors) who go out in various conditions, who are very skill focused. Other groups much less so, I'll leave it at that.
You will find them formally as clubs w. officers.
Some are under the ACA umbrella as Paddle America Clubs. Others are non-ACA. The Paddle America designation, among other things, allows the club to purchase insurance for on the water events. They are able to provide ACA instructors w. insurance at classes and symposia. In return they agree to adhere to ACA guidelines, for example, PFDs on whenever on the water. One of my clubs is a Paddle America club of 21 years standing. I get more open water experience via this club.
Other clubs, nonACA, also organize classes acc. to group interests and member skill levels.
Some groups or clubs are tilted very much toward seakayakers, altho member may own and enjoy other paddlecraft. Others have members in rec boats, SOTs, surf kayaks,canoes etc, as well as seakayaks. The composition of the group often dictates what types of paddles are organized. My second club is like the latter, I enjoy the variety and get in more river paddling via this club.
I left one yahoo group bec. IMO they were lax about PFD use and other safety issues, and there was too much alcohol before and during paddling. I didn't want to be part of that in the event of a serious accident or loss of life.
I belong to a second yahoo group that is the opposite.
You'll just need to jump in and see what local groups align w. your own interests and aspirations. Many groups and clubs are delighted to have (properly prepared) guests join them for a paddle or two.
Another benefit is the opportunity to try different boats, to get in on good deals when boats and gear go up for sale. You can ask questions about paddles, paddlewear, etc and see it in use. And you may find a few close paddle buddies that like the same paddling venues and conditions that you do.