SQL Server: How to know when a table has been queried

I am working with a database which exists off a commercial product.  It has "Group" tables that are populated when users add groups via the front end, and they may use their own groups or groups that other users have made.  I have a large client who would like to remove unused groups.  Nothing changes in the tables when the groups are being used.

Triggers don't work on the "select" command.  Is there a way that I may add something (back-end only, sadly) that would keep track of when a group was used?

Thanks
jr