This is more a FIY alert for people making travelling plans for Netherlands.
NS (the major rail operator) has this relatively new product: a group travel ticket that allows up to 4 people to travel on weekdays after 9AM and weekends all days, with exception of 3 holidays, for the relatively low price of € 49.
For larger groups, group tickets are available for extra fee of € 2.50/person (10 is the maximum)
It starts being a good deal for 3 or more people travelling together on day-trips.
The product is called NS Groepsretour