Do I need to grant system_level permission for report viewers

I would like to grant a windows group view reports on a SSRS report server.
They can navigate the folders and view the reports.

I know I need to setup item-level permission to the folder and give them browser permission.

My question is: do I also need to add them to system-level as a system user?
Is that a required step for permission to work? It seems I don't need to add them as a system level user, they still can view the report.

But in microsoft article:http://msdn.microsoft.com/en-us/library/ms156034(v=sql.105).aspx

It says:

The two types of roles complement each other and should be used together. For this reason, adding a user to a report server is a two-part operation. If you assign a user to an item-level role, you should also assign them to a system-level role.

This seems contradict with what I did. How can I unstand this above?

Thanks

SQLFriend