To add users (Administrators only), first go to Account on the left side menu.
Click on the Team Members tab and then you will see My Team. On the right, you'll see a button to + Add Team Member:
Click this button to add a new member to your team. You'll need to input the first and last name and email address of the person you are adding. Also, indicate if you want them to be an admin with the ability to change group settings and add other team members.
After they are added, the new member will receive an email to verify their account and choose a password. All done!
If adding a new member adds a seat to your subscription, you will be automatically billed on a pro-rata basis.
To deactivate a user, click on the user in the list to edit and un-check Active. This is also where you would set or remove the user as an Admin.