If you make reservations for several persons they normally get seats together if that is possible. 

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website.  You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new topic, and you will get advice.

Please note that Interrail/Eurail charges an extra fee of 2 EUR per person and ticket in addition to the fee for the seat reservation.

Well-just as you would do for a trip in this hexagone of yours with that national society to run les trains: go to counter. or call on the phone.

But that may indeed sometimes be a hard thing to do for trips perhaps very far away.