Currently, when a Category is restricted access, it displays a ‘lock’ symbol. However, members who do have access may not know exactly who has access to the group.
One possibly solution would be to enable clicking on the lock and a pop-up box appears showing a list of all the members who have access to conversations in that space. This would be a merge of all the members of all the groups who have been granted access to that Category.
What I do is use the category description for that. If it’s TL1, for example, I explain that in the description so it pops up on mouseover.
I don’t love the lock icon for restricted access though. People understand “locked” to mean “inaccessible” (reinforced by the use of the lock icon on closed posts) – not “locked to certain people.”
Restricted categories are only displayed for users who can ‘See’ the category. This means that your users who cannot see the category will not even know that it exists. If you try viewing your site as a Trust Level 1 user, you’ll see how it works.
I’m not sure what the best approach is to let users know that they need to join a group to get access to more content on a forum.
There is nothing build into the UI that tells a user which groups have access to a category. You could add this information to the category’s ‘About’ topic. You could also add it to the group’s ‘About Group’ section.