How to make the referral log into the same server?

If the server has enough slots, and the referral has received the invitation mail, how to make the referral log into the same server?
Thanks a lot!

You can't really.

All people who make a new acct will start on one of the newest servers.
They can't pick the server.

Sorry man.

well, soon there will be a change server option, which can be used.

CURRENTLY: when you both reach level 10, you may each make a new account, that will be put on the next newest server, therefore, if you make the account at the same time, you should be on the same server.