From the Manage Users page, click on the Add a new user button, which will take you to this page:
Simply fill out the fields for each new user you would like to add. For tips on creating secure passwords check out this article:
You can then choose the accessibility level for the user. For an explanation of each accessibility level check out this article:
Once saved, you need to send the user their log in details. Remind them they can change their password (and other personal details) through the My Account tab.