How do I put a user in a group for the IsUserInRole trick in C#?

if (!System.Web.Security.Roles.IsUserInRole("Whatever"))
{

 
 

I can't get this to work. A coworker thought that the "roles" were really "groups" (under "Server Manager" at Windows Server 2008 at "Groups" beneath "Local Users and Groups" beneath "Configuration") but that's not panning out for me.