Which version of OBIEE are you using?
You mention a document about OBIEE 10g but tagged your post as 11g.
In 11g things works in a different way: you add users/groups to approles in Enterprise Manager. Look at the doc for the correct version of OBIEE and you have all the details (or also googling you will find tons of blogs about that).
I am using 11g and haven't found the similar doc for this version. Isn't there a way to assign users to roles in rpd itself. I am posting here because googling didn't help.
One of the main changes between 10g and 11g is that security has been moved from the RPD to Weblogic. So the management of application roles and their members is now done in Enterprise Manager.
In the RPD you see a list of application roles and users to allow you to define security or other limitations based on them but you don't define them anymore in the RPD.
Have a look here: An OBIEE 11g Security Primer : Introduction - Rittman Mead Consulting