Select "Manage License Keys" to head to your user profile.
Once you've found the license, click the grey cog icon to enter the license key.
Clicking the green plus icon will open a new dialog
Enter the users email address and click the green "Search" button, once you see the user, click on the blue "Select" button to assign the user. You will not have to do anything additional, once the user has been selected the user can go to their account and see the license straight away.