The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
@brutus5000 That doesn't make sense because we have been playing for years on the client, and once the account is linked to Steam it cannot be unlinked. He tried to link his account last night and it said it was already linked as well. But that's fair, please look further into the issue later! Have a good night.