I've never joined mid year, but there's always been someone who has joined my society mid year (either through refreshers or coming back from a semester abroad). If they have a facebook page, maybe either put a post on that or message someone on committee saying you'd like to join, that way they'll know to look out for you and you'll have some friendly faces waiting. go for it, throw yourself into it and after a few weeks, it won't seem like you joined late.