Go to your Game server > Console page. Avaliable commands:

whitelist on — Enables the server's use of a whitelist.
whitelist add USERNAME — Adds the player name to the whitelist. The player does not need to be online.
whitelist list — Displays all player names in the whitelist.
whitelist remove USERNAME — Removes the player name from the whitelist. The player does not need to be online.
whitelist reload — Reloads the list of player names in white-list.txt (1.7.5 or earlier) or whitelist.json (1.7.6 or later) from disk (used when white-list.txt or whitelist.json has been modified outside of Minecraft).
whitelist off — Disables the server's use of a whitelist.