Teammates in your account can have their roles upgraded from a user to admin. Users invited into the account can remove their own login at any time, but cannot remove anyone else from the team.
Anyone who is an admin can remove teammates from the account as needed, and can promote other users to become admins.
Click on the Account menu - manage users
If you're an admin you'll be able to promote other users and remove teammates from the account. Click on the drop down menu, select the role and click Change Role to update.
To immediately remove a teammate, click on the Remove User button next to their email.