NOTE: Only team owner can buy a User Bundle.
User is a common concept used to unite clients and team members.
Basic and Small Business plans have fixed number of users you can add to the team (3 and 10 respectively). Starting from the Standard plan, you can add User Bundles to your subscription (10 additional users for $100/month).
To add user bundle, go to the Admin>Plan page of your account, find your plan in the list and change the number of user bundles you have.
Then number of users available for the team will be changed immediately, your payment will be recalculated at the end of a billing period.