To add employees and/or independent contractors to your account you first need to open your My Account section. You'll find this by clicking on your logo in the top right of your screen, as shown below.
Then you have to scroll down and click on 'users'. There you can choose the subscription you want your employee or IC to have and press 'Subscribe' or 'Update Seats' at the bottom of that box. *Note* Prices shown below are indicative
After you have purchased your extra subscription(s), you can now allocate a subscription to your new IC or internal staff member.