Access to functionality is based on user roles:
Adding a new user
From the users list page click the "New" button, enter the new user's email address and click "Next". On the following page enter additional details for the new user such as their name and password and select their user role.
Manage existing users
You can view the list of users in your account on the users list page.
Only administrators and sub-account administrators have access to manage users and sub-accounts.
If you aren't an administrator, the user list page will show you the name and email address of your administrator(s). Contact them to have them grant you additional permissions or add new users.
You can see your current user role on your profile page.
Changing user roles
You can assign a new role to a user from the user settings page:
If you have multiple connected accounts (called sub-accounts) you can designate each user to have access to one or more sub-account without having to create new user logins. They will be able to switch between sub-accounts they have access to.
On the user settings page select the sub-account(s) the user should have access to and set a default. The default account is the account they access directly after logging in.