To create a new user click on "Users" in System Administration. Click the "New" button. Enter the new User's information. After you are done entering their information and options, you will be prompted for a default password. Create a password of your choosing, which you will then provide to the new user along with their username and the Internet Address and Installation Token for your installation.
Only designated Administrators at your company can access the System Administration screen -- if you don't see "System Administration" under your "Go To" menu (it is the last entry in the list, at the bottom), then you do not have permissions to access this area.