score:4
This can be changed on all routes excluding the Tallinn - Helsinki route according to the Terms and Conditions:
4) The passenger cannot change the direction and/or itinerary, the date and time of departure or the number of passengers less than 24 hours before departure.
Whereas for the Tallin - Helinski route:
4) The passenger cannot change the direction and/or itinerary, the date and time of departure or the number of passengers less than 45 minutes before departure.
I assume you will find the option to add passengers online or else you might need to contact them:
Email: [email protected]
Phone number: +49(0)40 547 541 222