As of now, if their is an empty slot in your tank any random person can get in. The only times someone gets in its either to
A. Drive around randomly on purpose to troll
B. Attempt to help by filling in for a dead or absent crew member but utterly failing because nobody uses a mic
option A happens far more often than option B
Long story short other players shouldn’t be able to enter your vehicle as it is never helpful to a tanker that doesnt have a full crew