I have already made a booking. Can more people be added to it?
You can make a new booking for your travelling companions online. There is no guarantee that they will be sitting together, though.
You can also make a new booking at an NS International desk in a Service Shop or with Customer Service on +31 30-230 00 23. A member of staff will book seats together, subject to availability. A € 7.50 booking charge per person is applied, with a a maximum of € 22.50 per booking for three passengers or more.