In my experience of actually doing this: USUALLY, when a group of people are all on the same
MDE, the computer screen I see after scanning one of their MagicBands or cards has a green arrow in the top corner with something like "1/5" (if they're one of a group of five) and I can tab through the entire group.
But, that's not always there, because MDE can be weird (to put it nicely). If all the tickets were purchased on the same transaction, and they were bought directly from Disney or as part of a package, or from any of the resellers that don't buy bulk tickets, I can pull up the transaction and see them all that way. If they were bought from certain resellers that buy bulk, the transaction will have, like, 5000 tickets in it, and that's not an option either. At that point, I'd have to ask the guest to pull up their MDE on their phone, get the visual IDs of the MagicBands or cards belonging to other people in their party, and use those to find their tickets. This is also the step I jump to if the tickets were purchased separately.
Long story short: It's definitely doable to upgrade everyone's tickets without their media present, but if the green arrow doesn't show up for the CM, they might not have the skills or knowledge to find the other tickets. If they tell you they can't do it, offer the visual IDs from MDE. If they don't know what to do with that, ask someone else.