My suggestion would be to us View > Machines and filter my Machine Group name. I tested this a moment ago and all the machines currently listed under the Machine Group I filtered on showed after implementing the filter. You would use the filter options Machine Group > Does Not Contain > Machine Group Name. A caveat here would be the View > Machine view is dynamic so machines can move about to other Machine Group containers if they are located in multiple groups. In other words, scan a machine in Machine Group A, it would show under Machine Group A. Scan that same machine from Machine Group B it will be listed under Machine Group B So if you do use this method, you need to make sure you run scan the Machine Group before verifying what machines are in the Machine Group.
There may be a method to pull this from the database, let me perform a little research and I will post and update as soon as I have one.
You could try using out Report Views: Report Views Guide
I believe off of [Reporting].[Machine] would give you enough information to determine what machine are included in your Machine Groups.
Let me know.