Varies per railway and per train. Impossible to tell without further details. In person on a station/counter it should give no probs to let 1 do it for all-also on most=not all sites of railways. I have no exper. with this app.

We are travelling through Germany, Austria, Italy and Switzerland.

In that case:

• Seat reservations are optional for day trains (except for Italy)
• In Switzerland, reservations are unnecessary and almost nobody makes them.
• In Austria, alse few people make reservations, although a bit more than in Switzerland, mainly for longer distances.
• Please don't use Interrail for seat reservations, since they ask 8€ per person and train. ÖBB ask 3€ per person and train and DB ask 4€ per person and journey (can include multiple trains in/via Germany). In both cases, choose "Seat only”. And the best is indeed for 1 person to make them so you're al placed together.

Thank you for your reply!

Additionally, reservations for day trains in Italy and between Switzerland and Italy can also be booked via ÖBB, not using "Seat only” but with these steps, without any booking fees.

But its also ok too just use Interrail for all the reservations with a mobile pass? So we know we sit together and if just feels safer and easier. 

If you book via Interrail or via other sites it is the same, they all use the same reservation system. For trains in germany you can also check the seat plan of the train on bahn.de and choose where you want to seat. For austria via ÖBB only for Railjet possible. 

But if we have to book 4 seats for a train from Innsbruck to Verona via ÖBB and we all have a mobile pass, do we just have to choose “seat only”? Is are mobile pass with those four booked seats via “seat only” enough?

Yes, choosing "Seat only” is enough, since you all already have tickets (your passes). You can book seat reservations anywhere you like and if you don't like paying extra fees, it's best to avoid the Interrail reservation service, since they add at least 2€ to every seat reservation.

By the way, reservation is not mandatory on the EC trains from Austria to Italy. This is an error in the app (Eurail/Interrail has been told numerous times), so you can also just board the train.

However, there is an additional pass holder supplement on these EC trains (10€ in second class). You can buy that in advance but the price on the train is the same and some ticket inspectors don't ask for it :)

Ok so you can buy these supplements in the train from the ticket inspector?

Yes you can.