I was looking around, but I could not find any way to fetch the groups that the user is in. The only thing I can retrieve right now is the primary clan id of the user. I would like to be able to see all the groups the user is in for my app.
Have you tried using the following feed?
http://api.steampowered.com/ISteamUser/GetUserGroupList/v0001/?key=your_api_key&steamid=[Steam64 id of user]