To create a new user you yourself have to be at the right level of access, if you aren't then you won't even see an option to add a new user. If this is the case, speak to the web master of the website as they can create a user and grant them permission to all or only specific sections of the site.
If you are a web master and you are indeed trying to add a new user, you will need to go to the 'Start' screen which can be accessed by clicking the start link just underneath the blue header. Once here you should see 'Website -> Assets -> Users' as options. Click 'Users' and you will see there is at least one pre made group to make it easier for you to just add them to a group and they will automatically inherit the permissions of that group. Bear in mind, if the new user only needs permission to edit and access certain areas you may need to make a new group with predefined permissions to add them to. If you are the web master, do not add them to your group (unless actually intended) as they will gain permission to edit the entire site.
Once you know which folder you're adding the new user to, you just need to fill out a name, email address and password for the new user. We always advise telling the new user to change their password when they sign in first time for security reasons. A user can edit their own profile using the 'Edit profile' option along the very top of the window. Email the new user with their details and tell them to change their password ASAP.