@pomstan @PhenomX6 @xianc78 @alex @gnusocialjp Either they reach out to instances they want to federate with, or other instances reach out to them.
This is kind of how Pleroma is already, as the public timeline is empty by default, and fills up as you start following or get followed by users of other instances.

Likewise, PeerTube is another example of that; the admin must follow another instance first in order to retrieve content from other PeerTube instances.