Ext: Usergroups Additive Groups

I recently discovered the userGroups extension.
I was surprised that a user can only belong to 1 group?!

I have a few datasets that tie together, so this would be really useful.
For example:

• Projects
  
• Purchasing
  
• Customers
  
• Suppliers
  
• Admin

I was thinking I could have groups like:

• Projects_Admin, Projects_Contributor (Projects)
  
• Contacts_Admin, Contacts_Contributor (Customers & Suppliers)
  
• Purchasing_Admin, Purchasing_Contributor (Purchasing)
  
• Root, Admin_Maintenance (Admin)

• So a basic user might belong to groups:
  
  • Projects_Admin + Contacts_Contributor
  
  
• and a Project_Manager might belong to groups:
  
  • Projects_Admin + Contacts_Contributor + Purchasing_Admin
  
  
• and the administrator would belong to groups:
  
  • Root (which has admin on all controllers)

I think I will need to extend the userGroups extension to allow for a user to belong to multiple groups.
Has anyone implemented something like this already?