It asks me for a (registration) token...?!?
We are currently running Acter in a private beta. If you do not yet have any Matrix account on any other server, you can register one with our server
acter.global. That however is currently limited to invited organizations. Whoever asked you to join should have received a specific token for your organization, please ask them for the token. We will lift this requirement as soon as we reach a public beta status.