How can I get the local group name for guests/administrators?

The names of groups are localised depending on system language.

For 'well known' groups like 'Administrators' and 'Guests' you should retrieve based on the SID. The SID for Guests is:

S-1-5-32-546

There is a list of well known SIDs here:
http://support.microsoft.com/kb/243330
Looking up the account by SID is the best way to go. It's a bit contrived, but the way it works is this:

The Administrator account's SID always starts with S-1-5-21 and ends with -500. Everything else in-between is random (the domain's SID).

The Guest account's SID always starts with S-1-5-21 and ends with -501.

System.Security.Principal.WindowsIdentity currentUser = System.Security.Principal.WindowsIdentity.GetCurrent();

var sid = currentUser.User.ToString().ToUpper();

if (sid.StartsWith("S-1-5-21") && sid.EndsWith("-501"))

{

return "Guest";

}

WindowsPrincipal wp =

new WindowsPrincipal(currentUser);

Console.WriteLine(wp.IsInRole(

WindowsBuiltInRole.Guest));

Console.WriteLine(wp.IsInRole(

WindowsBuiltInRole.Administrator));

Console.WriteLine(wp.IsInRole(

WindowsBuiltInRole.PowerUser));