Wait a minute, you have secret forums? We do. The fact that we have secret forums, not a secret. The forums themselves? Yep, secret.
Secret forums are invitation only. If you are not a member, you won't see it or its contents anywhere. We might use one so that people testing an alpha or private beta release can get the software, get help, and give us feedback before we go public, or even announce the existence of something. Hence.... secret!
To answer the question, three ways.
1: if you're invited, you'll get a notice via email with an invitation to join and instructions. Once you get to the secret forum, make a bookmark so you can get back easily. It's secret after all.
2: In the Avatar menu, choose Your Places. Any place (spaces or groups) that you follow will show up here, including secret places. You can filter to see just spaces, projects, or groups.
3: Search for it. This of course requires that you know the name.
In the search results, at the bottom, will be Spaces or Groups that match your query.