Collaboration is available to subscription plans with multiple seats. If you purchased the subscription, you are the administrator and can invite your colleagues to license photos on your Twenty20 subscription.
Click on your avatar (or username) in the top right corner of the navigation bar. Then select "Settings" from the dropdown menu:
Once you're on the Settings page, toggle to "Subscription":
Click "Invite" and enter the email of your colleague to add them:
If they have a Twenty20 account, they will be added to your subscription and notified via email. If they are a new user, they will be invited to join Twenty20 and added to your subscription at the same time.