See if user is part of Active Directory group in C# + Asp.net

With 3.5 and System.DirectoryServices.AccountManagement this is a bit cleaner:

public List<string> GetGroupNames(string userName)
{
  var pc = new PrincipalContext(ContextType.Domain);
  var src = UserPrincipal.FindByIdentity(pc, userName).GetGroups(pc);
  var result = new List<string>();
  src.ToList().ForEach(sr => result.Add(sr.SamAccountName));
  return result;
}

The code below will work in .net 4.0

private static string[] GetGroupNames(string userName)
{
    List<string> result = new List<string>();

    using (PrincipalContext pc = new PrincipalContext(ContextType.Domain, "YOURDOMAIN"))
    {
        using (PrincipalSearchResult<Principal> src = UserPrincipal.FindByIdentity(pc, userName).GetGroups(pc))
        {
            src.ToList().ForEach(sr => result.Add(sr.SamAccountName));
        }
    }

    return result.ToArray();
}

Brandon Johnson, loved it, I used what you had, but made the following change:

private static string[] GetGroupNames(string domainName, string userName)
{
    List<string> result = new List<string>();

    using (PrincipalContext principalContext = new PrincipalContext(ContextType.Domain, domainName))
    {
        using (PrincipalSearchResult<Principal> src = UserPrincipal.FindByIdentity(principalContext, userName).GetGroups(principalContext))
        {
            src.ToList().ForEach(sr => result.Add(sr.SamAccountName));
        }
    }

    return result.ToArray();
}