You have an account page, that is accessible in your left nav under "Account" but is also at /core/dashboard/accounts. It looks something like the below
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To invite someone, simply click the "Invite Someone" button at the top. To change the role of any user, you can hit the dropdown that shows their current role and change it.
See someone's name you don't recognize? If you are an enterprise client, it's possible a QA person on the RebelMouse team, a developer or your account person has access in order to better help you. If you are not sure why someone is an admin, feel free to ask your account manager and they'll let you know that person's role.