Basic and Standard Plans are single-user plans. Additional users cannot be added to these plans.
The Premium Plan includes 3 users. You can add 1 additional user to a Premium plan for a total of 4 users.
The Enterprise Plan includes 5 users. You can add unlimited additional users to an Enterprise plan.
The cost for each additional user added to a plan is $15.00. So, for example, if you have an Enterprise plan and you want 10 users, you need to pay for 5 extra and the cost is 5 x $15 = $75.