As CarolA posted, it doesn't matter (or at least it didn't used to matter). I made the airline reservations for me and two other adults and the only contact information Delta had was for me. There was absolutely no problem. I did receive the receipt and itineraries (and subsequent changes, LOL) for all 3 of us, but that was not a problem.
FWIW, this may become a problem in the future. The government is apparently working on some new screening system that requires the airlines to share passenger data with them. My understanding is that they(government) will do some kind of check to verify the identity of airline passengers. Not sure if that will require contact info for all parties or not, but if I had to guess, I'd say it will. The people whose identity cannot be properly verifed will probably either have to go through additional screening or maybe even be denied boarding.