In general, If it has something to do with "watercolor", or "painting", then join. If it has something to do with "photography", then join. It's that easy. If for some reason you later feel it was a mistake, just click the "quit group" button.
Just make sure you read the group description for particulars. For instance, if it's for abstract only and you don't do abstract, then obviously don't join.
What I'm most concerned about is that less and less groups will permit more than one image to be uploaded in 24 hours (even though they now have a "holding tank" of sorts). If I have a few images that I want to upload, I want to do it all at once. Not spread out over several days or weeks.