Thanks folks, I’d considered both these options - also came across a plugin but it’s been inactive for the past 2 years so unsure if it would still work: GitHub - rilla/discourse-show-private-categories: Discourse plugin to allow users to see categories read-limited categories and request access to them · GitHub.
I think I’ll have to set expectations that people use ‘groups’ to find the interest groups which are available and set them all as allowing people to join. At least with a bit of user-education people should be able to figure that out!
Ruth