I tried to join the Ultimate Frisbee society in my first year. Unfortunately training was on a Wednesday (when I work), a Thursday afternoon (when I had lectures), and on a Saturday (when I work). You had to attend at least two of these, and since I could attend none I kind of couldn't do it. I did go to the taster session though and it was bags of fun.
I also paid to join the Monster Raving Looney society two years running (costs cover a very robust top hat), but I've yet to get round to going to a meeting. This is largely because early on in both first semesters I had a lot on so missed the first set of meetings. I would've felt weird going to a later one and not knowing anyone, so I didn't. I also joined the Anime society but never went.
I've loved Uni but my biggest regret is not joining a society.
house won this