How to add new Guid to a String List?
public Guid Login(string userName, string password)
        {
            Guid result = new Guid();

        if (!string.IsNullOrEmpty(userName) && !string.IsNullOrEmpty(password))
        {
            userName = userName.Trim();
            password = password.Trim();

            Member member = BusinessDb.LoginDbJobs.GetUser(userName);

            if (member != null && member.PasswordDb == password)
            {
                result = Guid.NewGuid();
                Members.Live.Add(result); //--->Object reference not set to an instance of an object.
            }
        }

        return result;
    }

if (!string.IsNullOrEmpty(userName) && !string.IsNullOrEmpty(password))
        {
            userName = userName.Trim();
            password = password.Trim();

            Member member = BusinessDb.LoginDbJobs.GetUser(userName);

            if (member != null && member.PasswordDb == password)
            {
                result = Guid.NewGuid();
                Members.Live.Add(result); //--->Object reference not set to an instance of an object.
            }
        }

        return result;
    }

...
    public class Members
    {
        public static List< Guid > Live;
    }
...

public class Members
{
    public static List<Guid> Live = new List<Guid>();
}

public class Members
{
    public static readonly List<Guid> Live = new List<Guid>();
}