For Italy it’s best to use tickets.oebb.at as there’s no booking fee. But it only works for groups of 6. So either you split your group in 3 : 6 + 6 + 5 and you might or might not sit together.

I’d advise to contact Trenitalia, they should be able to help you : https://www.trenitalia.com/en/purchase/info_contacts.html

Otherwise you could become a Eurail “partner” (don’t know how it works) : https://www.eurail.com/en/about-us/become-our-partner (see institutional partnerships)

Hello,

I can see my customer service colleagues have replied your question regarding the reservation. As what they wrote, you must split the order into separate bookings due our system only allows to book reservations for up to 9 travellers.

Have a nice day!