as much as i like the idea, i'd much rather find someone to ride with on my own, in person, from my own social circle. that removes the reliability and trust issues.
We are currently shying away from another facebook app, but maybe that's the way to go for this.
the code feature could work to solve the issue, yes. or you could just allow drivers to administer how public their rides are, only allowing people to join if they are directly given the link to their ride or whatever.