To add a user, click the "Site Administration" button on the Main Page in the far left menu (Note: The Main Page is the Family Accounts page and is usually the one landed upon after logging in):

 Then click the "Users" button near the top right:

This will bring up a list of users. Above the list is an "+ Add User" button:

Click this to add a new user.  After filling in the details, click the "Submit" button:

This will bring you to the list of users again. Search for the user you just added, and click the "Reset Password" button next to that user:

This will send an email to the user with a temporary Password and login instructions.