Pulling the users of an Active Directory groups

using System;
using System.DirectoryServices.AccountManagement;
using tht.EDRMS.Service.Interfaces;

namespace tht.EDRMS.Service.Services
{
    public class ActiveDirectoryService : IActiveDirectoryService
    {
 

        public string GetAdGroupMembers(string groupName)
        {    
            var users = "";
            using (var ctx = new PrincipalContext(ContextType.Domain, "tht.local"))
            {
            
                GroupPrincipal grp = GroupPrincipal.FindByIdentity(ctx, IdentityType.Name, groupName);
                if (grp != null)
                {
                    foreach (Principal p in grp.GetMembers(true))
                    {
                        UserPrincipal up = (p as UserPrincipal);
                        if (up != null)
                        {
                            users += up.EmailAddress + ", ";
                        }
                        
                    }
                    grp.Dispose();
                }
            }
            return users;
        }
    }
}