Best way to keep and be able to discover list of people for example from the various TwentyOne (Dvadsaťjeden) groups?

should we create a Nostr key to manage the group in each city and keep a list under that key? What service to use?

I would like the list to be easily usable (one click follow all, or manually follow each) and as client agnostic as possible.

I realize we have a bootstrapping problem. When we go to a Bitcoin meetup together, I want to keep in touch with the whole community.

How to make it as simple as possible for everyone?