Seat reservation for multiple travelers

In short: some acquaintances of mine are organizing a 11-days japan trip through a travel agency, including a 7-days green pass for each of them (not really needed imho, considering their itinerary, but still...)
I'm helping them since I've been traveling in japan for a long time, but still my JR pass use was always by myself or with a single partner...If they plan to book 4 train tickets for the same train/time, how is the best way to proceed?I used the new JR pass a few times at the ticket machines, but is it possible to book multiple seats with a single JR pass or every passenger has to make a separate reservation?Otherwise, maybe is it easier to make a booking if they go all together at a manned ticket counter?

Thanks in advance