Welcome to BCUK!
It isn't very clear and the groups software is a bit different from the rest of the forum (for example email notifications don't work at all) so you need to keep an eye on it.
Near the top of most pages there's a dark bar with a bunch of not very obvious links. Click on 'community' then 'groups'.
I usually rely on my browser's memory and type a few unique characters in the location bar instead of using the site's, er, rustic navigation facilities.