So let's say: 
- I set up a group chat for my normie friends (Uncle Jimming a relay+blossom server for them). 
- I run a lightning node (Alby Hub style) 
- I run a #cashu mint on top of it

How do I now: 
1. only let that private group use the mint?
2. maximize privacy?

Is there some kind of "Proof Of Group Member" that they can share?
Is there a way for me to run the mint in "Blind Mode" or something (and prove it to them!)?