Move users within subdomain

You don’t have to transfer them . just add them. Sub site admin -> Users -> Add existing.

users are global to the network – if you just want them to leave comments… they already have that.

I cannot find “Add existing”.

I have installed wordpress on the main domain (in this example: https://www.google.com) and then created a subdomain (mail.google.com) installed a new wordpress installation in that domain with a different database.

Have I done this wrong?

No, but what you did is install two separate WordPress instances.

https://codex.www.ads-software.com/Create_A_Network

That’s MultiSite ??