Private groups visible when you click the "Invited" link as a public user

One of my users give someone on another site this link

http://city-builders.info/cb-groups/invited

and suggested they click it, when I checked as a public user I found that I could see about half the available groups on the site, including two private (invite only) groups, you cannot access either group so its not a major problem ... more an annoyance rather than anything else and as such I can live with it, but at some point when you have some time to spare if you could take a look I'd much appreciate it.

thanks