Your friends have a few options for cheap golf.
They can join the golf society for 5 e and get discounted golf on mon, wed and fri mornings.
They can play after 2pm on a twilight ticket ( get them to check this time in pro shop as it varies with season)
Other times they must provide proof they are renting from you so a letter of introduction and a copy of a utility bill from yourself will allow them to play on owners rates.