This is the most important decision you have to make when you think on going in a InterRail.
You will be side by side with your friends 24/7 during the time of your InterRail. Make sure they aren’t too afraid, or if they don’t create too much fights or arguments (even if they are you’re best friends / girlfriend / boyfriend). It’s no fun to have arguments and fights during your trip.
If you take a big group fights will happen and the group might split up.
The “common sense” is to go to a InterRail with a group with a maximum of 6 people.