The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. Then you can make reservations for as many seats as you need.

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. 

1.may be-sometimes-depends on railway and type of REservat. Those of 4-55 for seat only usually not.

2.by not using app and the ways Anna links to-as apparently the major frightening aspect of this is to ´not sit together´ for a few hours-then its best done at a counter.