When in the Content Administration Tool, go to “Users” in the left side menu. Click on “Add user” and type in the email address for the person you want to invite. Click on the + icon to add the user to the invite list. You can also fill out their first and last name. Choose the role you’d like the new user to have. You can add more than one person to the list before clicking “Send invite”. The users will then get an email with sign-in credentials to use.
Read more about the different user types here: https://www.pickit.com/help/what-different-kinds-of-user-types-are-there