Out of the box, accounts for the platform are set-up and configured for specific roles by the tenant administrator. See the roles and permissions table for more information on the different types of roles.
If you want to add or remove a user, you can do this (if you have the proper rights) in the admin section under the user tab.
Add users
Use the Add user button to add new user to your tenant
Remove user
Click on the remove icon (x) behind the username in the user tab to remove the user from the tenant.