Users added for a company can broadly be divided in 3 ranks.
Super administrator
The person who signs up for the company account and sets it up, is treated as super administrator.
A super administrator can perform all the operations possible for a company and is the only one who manage the billing plans and subscription for the company.
Any other user can be upgraded to have the rank of a super administrator, but there can only be one super administrator for a company account.
Administrator
An administrator can :
Change general settings
Limited rights user
A non administrator needs to be added as a member of a project for them to do anything in a project.
What operations they can perform is determined by the role they are assigned.