If we travel after 9:00 and return after 18:30, three people can travel with me on 40% discount - possible if you have an OV-chipkaart. That card costs €7,50 and you need at least €20 on it to travel by train, so getting one just for this trip is not worth the trouble. But if you have one already then you can use it to travel on discount with me.
For those who don’t have that card, we can indeed get a group ticket. @Stefan is right in that one person needs to order for everyone. I think however, that every member of the group gets their own e-ticket, because you need the bar code to open ticket gates. (edit: yes, this is true, all group members need to have either the app or a printed version of their ticket.)