You can manage users by clicking the ACCOUNT link near the top right and choosing USERS:
If this option does NOT appear, you do not have user privileges for your login.
Clicking on the ADMIN GROUPS tab will display your current group(s) of users (the "admin" group is the default and cannot be deleted). Every user MUST be part of a user group, and you can administer users by updating permission settings for an entire group of users at once.
Once you have created/editing the necessary groups, click the USERS tab and create a new user:
When adding a new user, you must select ONE group from the pick-list:
At any time, you can edit permissions for an entire group of users by clicking the ADMIN GROUPS tab, and then EDIT next to the group:
You may select MULTIPLE lists for each admin group on this page.