I actually think Lemmy should take a page from Mastodon here. Instead of users creating groups (which sounds like a huge headache to implement smoothly), Lemmy should add hashtags or something similar. So I would sub to #photography and people posting would be restricted to a max number of hashtags (TBD). You can then choose to stay subbed to the hashtag and/or sub to the communities that crop up frequently on your feed and then unsub from the hashtag if you want.