You are only eligible to have one passenger upgraded with you to C+. You will need to split the PNR so there is only you and a single companion on yours (otherwise no upgrades will be processed). All passengers in a PNR must be booked at same time with the same fares and itins. Any attempt to modify one of the passengers tickets will require splitting them off the PNR onto their own. DL upgrade section says General Member companions on separate PNRs are not eligible for Medallion companion C+ upgrades (only FC upgrades by linking), but not entirely convinced this is true (I believe some have reported success in getting companions on separate PNRs upgaded to C+ by linking). At any rate, you can still only upgrade a single companion. C+ upgrades are not merely a seat selection option as C+ is a separate fare class on DL. When upgrade processor upgrades a pax to C+, their listed class get changed to SU (just like FC upgrades change your class to OU). Theoretically, they could limit SU bucket inventory to less than available seats for sale (as they do with OU bucket and FC seats). Although that generally doesn't seem to be the case.