When reading the FAQ and different parts of the website, the provided information is quite confusing, it might be possible to add passengers for free, depending on your fare type.
For one, they mention that any change for the Dover-Dunkirk route should happen on the phone and that there will be a cost of 10GBP/15EUR plus the difference between the former and new price (depending on the day you change your booking)
But at the top of the website, there is an "Amend" link and the details of possible operations include
Amend passenger details, date/time of travel, vehicle type for routes from Dover to France. There is a help section about this amendment feature and it says
simply go through the same booking process that you did when originally making, which suggests you can change the information you filled in the first place. It mentions that before the booking is actually updated, you would have to pay the possible extra charges.
On top of that, they offer two fare types, and clearly state the Economy type has a 10GBP charge for booking amendments, but not the Flexi type. So most likely, any change would result in a 10GBP charge.
If you already made a booking, I would recommend you to try the amendment tool right away and try to simulate your changes, to see if you can actually amend your booking and if they charge you any extra.
If you have not made a booking yet, try to ask your friends to make a decision before you book, since the amendment is likely not to be free if you book the "Economy" fare. Note that if you book at least 2 days in advance (e.g. a Wednesday for the Friday), prices do not fluctuate a lot. They seem to change from 35 to 65 GBP, probably depending more on the time of the week/time of the year than how early you book. In particular the 10 GBP charge seems to matter more than or at least as much as the price fluctuation (I have not run a deep analysis of the price evolution but prices 2 days away can be as low as 35 GBP, except on Friday evening, and prices a month away can be as high as 65 GBP)