title: ActiveDirectory DirectorySearcher: Check if user is member of a group
body: <p>I want to know if a given user is member of a group or not. Now, I don't know much about ActiveDirecory or Exchange servers, but in Outlook I can see that a user can be "memberOf" a group (and i can query those groups with <code>DirectorySearcher.PropertiesToLoad.Add("memberof");</code>), but there are also other groups that users are not actively <em>members of</em>, but that <em>contain</em> users. If you mail to those groups (or aliases) you reach all the users contained in it.</p>
  
  <p>Basically, given a username (like <code>DOMAIN\JDoe</code>), how to check if it is contained in the group <code>FUNNY_USERS</code> in C#?</p>