Actually this is a pretty nice idea, and coupled with the future
enhanced user groups, would make it a great addition.
I would add it as a function to the plugin core and make a note for it in the Groups description, that you can manually hook it to the theme, anywhere you want the user group name to appear.
For example, if you create the hidden group
Sellers and add a user as a member of that group, you can hook the function to the User Public Profile, so everyone would know that the user is an authorized Seller.
Sweet!