You can have a look with the sample in MSDN with following URL too:
ms-help://MS.VSCC/MS.MSDNVS/cpref/html/frlrfsystemdirectoryservicessearchresultclasstopic.htm
Something like this:
using System.DirectoryServices;
...
string adPath = @"LDAP://Domain.com/CN=user1,CN=Users,DC=domain,DC=com";
DirectoryEntry user = new DirectoryEntry (adPath);
foreach(object group in user.Properties["memberof"])
{
Console.WriteLine(group.ToString());
}