Hello Marzipan,
Thank you for your continuous interest in our products! :)
The ADWS GetMembersOfGroup method indeed only returns the members of a specified group. I think that you may be able to accomplish returning all the groups by creating a dummy user account in Active Directory and adding that user to all of your groups. After doing this, you can then use the GetGroupsForUser method specifying that user's alias in the query.
Hope that works for you.