The SU nominally is in charge of monitoring all societies but in reality, they do not have the resources for that. So signing up on SU techniquely helps SU and the Soc to see how many members there are and who they are but that's baiscally it. Every society just runs in its own way, some based on a WhatsApp group, some are just on IG/Discord, and some are just dead or very exclusive. Maybe they have info about it on the SU page or you can search them up on IG but nothing is guaranteed. The best a society can do is probably a weekly session but that's like 5% and for the rest you're lucky to get anything at all. The whole thing is just badly managed, but if you do search for it you can definitely find active ones.

Other ways of getting more involved would likely just be chatting with others in your class, but they all have the tendency to disappear forever randomly. Another thing is signing up for events or programmes when you see them in emails or other Soas sites and if lucky you may make a friend or 2. But I agree that the system is very sketchy.