After going through my browser history, I found it was hippoGROUPS, not Subscribe-O-Matic, that allows you to manage your subbed groups. Here's the URL: http://www.hippo-tech-sl.com/myhippogroups/
If it's S-O-M, have a look at this thread: https://blogs.secondlife.com/message/66523#66523. Sounds like group management was in the works..