Setting up security for SSRS in SharePoint integrated mode can be a bit tricky, particularly if you want to set up some of your users to only be able to run reports, but not to be able to modify or change them. I found a great post explaining how to create a Reporting group in SharePoint:

http://agilebi.com/cs/blogs/jwelch/archive/2009/07/10/view-permissions-for-reporting-services-in-sharepoint-integrated-mode.aspx