At the very beginning you get a free 14-day trial. It means that for the first 14 days you can test the system with no feature limitations, adding as many users as you want and have full support.
After the trial, you can choose among 3 subscription plans to sign up for.
When you choose a Professional, Professional Plus or Enterprise plan, you will be charged for each user in your workspace.
Imagine: you have 5 users and want to subscribe for the Professional plan.
You have two options: subscribe monthly or annually.
If you take a monthly subscription, you will be charged $30 per user which is $150/month/5users (5*30=150).
If you go with the annual subscription, you get a 20% discount, resulting in $24/user/month. This will account to 5*24=$120, saving you $30 a month and $360 a year!
If you decide to add a new user during the period of use (a month or a year), you will be charged the difference with the next payment.
Let's say you signed up for a year with 5 users and paid $1440. After 3 months you add the 6th user (they will use the system for 9 months before the next billing date). This way you will be charged for all the 6 users when the billing date comes, yet you will only pay the difference for the 6th one (for 9 months).
On the other hand, if you remove one of the 5 users during the subscription period (a year), the not-used money for their account will go to your credit balance. For instance, if they used the system for 3 months, you will get the rest, which is 9 months, credited to your account. You may use these funds for further payments.