Who gets in, who commands, who stays out, the Players page holds the four lists that govern access to your server, with no command to type.
Server players
The page knows the players who have been on your server and offers them directly, a nickname, a click, and the action goes to the server as if you had typed it in the console.
Four lists, four tabs
The operators get the administration commands, only put trusted people there, and remember that their exact power is set on the Settings page, tab "Operator level". The whitelist, once enabled in the World Options, only lets in the nicknames you put there, it is the main setup for friends-only servers. The banned by nickname and by IP close the door the other way, unbanning is done in the same place, with one click.
All these actions apply immediately, while the server is running, no need to restart to op someone or lift a ban.