As far as I know the SimpleX team adds their servers to the preconfigured list of servers in the app.
So if you host SMP servers for others, you could share their QR code (along with a quick description) in some forum or your own website (I know a hoster of serval private services who also maintains SMP nodes).

For info about hosting such a server, see here.

I see what you mean, so let me try to explain:
You don’t have an ID for the account (like a username, random number or even public key) but instead an address for your chat with someone (this is the queue where your chatpartner sends their messages to). So every address is unique only per chat, no unique ID per account or so.

If you still have questions about the SimpleX system, feel free to ask and I will give you useful links.