Participants in the private beta can invite other experts in. The "invite fellow experts" section in the right-hand column of any question allows you to invite people to join before the beta becomes public. It only requires their email address.
If you wish to invite others to this site, please use this link:
https://cardano.stackexchange.com/users/login?ssrc=beta
(at first, I thought only people that committed can join the private beta, but it seems like this link works for all)