Since you're on .NET 3.5 and up, you should check out the System.DirectoryServices.AccountManagement (S.DS.AM) namespace. Read all about it here:
Managing Directory Security Principals in the .NET Framework 3.5
Basically, add a reference to the assembly System.DirectoryServices.AccountManagement, and then you can define a domain context and easily find users and/or groups in AD:
using System.DirectoryServices.AccountManagement;
public List<GroupPrincipal> GetGroupsForUser(string username)
{
  List<GroupPrincipal> result = new List<GroupPrincipal>();
  // set up domain context - if you do a lot of requests, you might
  // want to create that outside the method and pass it in as a parameter
  PrincipalContext ctx = new PrincipalContext(ContextType.Domain);
  // find user by name
  UserPrincipal user = UserPrincipal.FindByIdentity(username);
  // get the user's groups
  if(user != null)
  {
     foreach(GroupPrincipal gp in user.GetAuthorizationGroups())
     {
         result.Add(gp);
     }    
  }
  return result;
}
The new S.DS.AM makes it really easy to play around with users and groups in AD: