You can activate a license key provided to you by another colleague to use Pro. If you already have Pro, you can use the key to extend your current subscription. For Basic users, this will activate Pro for your account for a year at the time of activation.
Login to Socrative and click on your name at the top right corner and go to Profile.
In the Account header, you can view your account details. If you already have Pro and are looking to renew, click on the Renew License button. If you're using the Basic version of Socrative, click on Upgrade.
Left: Pro user. Right: Basic user.
The link will expand into a new menu. Click on Enter License Key:
A new menu will show up to allow you to enter your license key:
Enter in your license key and click Activate to start using Pro!