So my friend and I want to play together but when he is registering on the website, it says this: To create ETS2MP account you must have ETS2 assigned to your steam account. If you have it in your steam library try to check if your profile is public. If it's private make it public and press F5. He does have Euro Truck Simulator 2 in his steam library and his profile is public so we have no clue what to do now as I didn't experience this issue while registering.