Whitelisting your server prevents users that are not on the list from joining your server. By default, your server is not whitelisted. So you must enable it and add users to it yourself.
To turn on your whitelist, log into Multicraft, select and start your server and follow these steps:
1.On the left hand side of your Multicraft control panel click the "Console" link. 2.In the console type "whitelist on".Make sure your server is on when you do this. 3.Now your whitelist is on, we should add a user. 4.To add a user type into the console the command "whitelist add USERNAME" 5.That's it! You can find many commands for the whitelist on the Minecraft commands page here.
Remember to whitelist yourself, you are not whitelisted by default!