As far as I know existing characters can't be transfered to other servers at release, right?!
We will have to stay on our old worlds (with a new server structure, but same community and such).
Because of this our/your ls members don't have to select a world at all.
Only new characters can be made on any (old and new) world.

That means all those characters already existent just stay as they are. And if you have friends who want to start playing FFXIV with ARR release then tell them to create their characters on your old world, too.

We'll have to wait some more time (after release) till we can transfer to other servers. And then we'll already know which worlds were created, and we can tell the destination server to everyone ingame.