User access is done via ssh's authentication scheme. As stated above, this is how users get access to the system, while maintaining a high level of security. If you're not familiar with ssh, check out http://www.ssh.org/. Note that I am using ssh version 1, not version 2. There is a big difference, notably that version 1 is free, and 2 isn't.
You'll need to configure sshd. The idea is to disable password authentication and rhosts authentication. The following options should be present in your /etc/sshd_config file.
PermitRootLogin yes IgnoreRhosts yes StrictModes yes QuietMode no CheckMail no IdleTimeout 3d X11Forwarding no PrintMotd no KeepAlive yes RhostsAuthentication no RhostsRSAAuthentication no RSAAuthentication yes PasswordAuthentication no PermitEmptyPasswords no UseLogin no