You can add and remove people from a single stack within the stack settings. Just click on the cog icon next to the stack title. In the list “Who can access this stack” you can add or remove people.
To grant admin rights or delete a user from the whole account you can do so by clicking on the “People” link in the top right menu.
If you grant admin rights to someone he or she will be able to access all stacks including the stack settings. Admins will be able to create new stacks, delete stacks and access all stacks within an account. Admins can also create other administrators.