Make it look like a centralised system initially. Provide a portal to a pre vetted/chosen instance that is accepting new members in their locale/country, that is the same for everyone.

Update: This (above) is badly written. I'm trying to say every potential new member gets presented with the same (pretend centralised) portal that is in fact an (valid long-lived) instance local to the individual potential for them to sign up with. So two local users in Oz get given a proxy to the instance local to them, and a user in Blighty an instance local to that person. The decentralised Lemmy looks centralised, but isn't. The proxy front end should explain that they're joining their local instance and it's like a network of little affiliated clubs that can see each others posts globally. they log in for the first time it will become clear.

It's late, I'm tired, sorry everyone. Is that any better?

I think it's confusing (the reverse of what they're used to) for a newbie who have been bought up in a centralised internet with single front ends of all the big players to be presented with little instances to join to access the whole.