You can request an account on this server by sending an email to both mieum _at_ tilde _dot_ team and slugmax _at_ sdf _dot_ org with the subject 'Republic new user request' and containing your requested username and SSH public key. You'll get a message back after some time (hopefully within a day or two) with confirmation and brief instructions on how to login, either using ssh or sftp, and how to get a hold of your account password (not required for login but still required for shell users who run chsh, for example).
You can generate a new SSH keypair by running 'ssh-keygen -t ed25519' and accepting the defaults on whatever host you want to connect from. This will generate a file named 'id_ed25519.pub' that is what you send by email. Take care to not send your private keyfile (the one named 'id_ed25519' by default), otherwise you'll need to regenerate both keys. Unless you are comfortable with plaintext email, it's also best to send the key as an attachment, as many email clients will mangle inline text.
For existing users on the other Circumlunar outposts, you can have a user account on only one of the outposts at any given time.