If you want to make your Rust server private and only allow certain players to join without having to remember a password, a whitelist plugin is meant for you. Rust currently does not have a built in whitelisting feature, so you can use an Oxide plugin to get the job done.
Steps:
1. Enable Oxide on your server.
2. Add the plugin Whitelist.cs
and restart the server.
3. Join the game or via RCON run oxide.grant user STEAMID whitelist.allow
*Note: Replace STEAMID with the player's Steam ID.
Your server will now be whitelisted.