There are a few different types of users, some are paid for, others free. See below to see which are which.
Administrators (Paid user)
God like figures who can do absolutely everything.
Staff (Paid user)
Can do most things but cannot add new users or close down tasks
Partners (Paid user)
If you work closely with outside companies you may want to give certain of their staff access. These partners can add new tasks, comment on existing ones etc
Contributors (Free user)
If you just want to add someone who can comment on tasks then use this free user type.