Demo code, ;-)

using System;
using System.Collections.Generic;

namespace houseParty
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> partyGuests = new List<string>();
            int n = int.Parse(Console.ReadLine());

            for (int i = 0; i < n; i++)
            {
                string[] guests = Console.ReadLine().Split();

                if (guests.Length == 3)
                {
                    AddGuest(guests[0], partyGuests);
                }
                else if (guests.Length == 4)
                {
                    DeleteGuest(guests[0], partyGuests);
                }
            }

            foreach (var guest in partyGuests)
            {
                Console.WriteLine(guest);
            }
        }

        static List<string> AddGuest(string name, List<string> partyGuests)
        {
            if (partyGuests.Contains(name) == true)
            {
                Console.WriteLine($"{name} is already in the list!");
            }
            else
            {
                partyGuests.Add(name);
            }

            return partyGuests;
        }

        static List<string> DeleteGuest(string name, List<string> partyGuests)
        {
            if (partyGuests.Contains(name) == true)
            {
                partyGuests.Remove(name);
            }
            else
            {
                Console.WriteLine($"{name} is not in the list!");
            }

            return partyGuests;
        }
    }
}