Never used this mobile thing, so DK.

BUT-as we have explained here so many times; about any REServation can also be done locally or sometimes online via the site of country/company concerned or even (so antique that many do not even realise this still existing waytodo: at the counter)-then all your desires can be met much better. In fact in some populair countries it has to be done that way!

If you book the reservations together, then the reservation system will place you together if possible.

For booking together using the Interrail reservation service, check this page. There are more ways to book reservations though, see e.g. this overview (not complete, but shows some alternatives).

Interrail charges a 2 EUR extra fee per person (so that's 10 EUR for your group) and train and 9 EUR postage fee per order if they have to send something via the post. Until now, I've managed to avoid using it.

Some more tips to start with:

• Request a pass cover number via this form if you buy a mobile pass. Some online booking forms need this (e.g. SJ). The paper pass has this on the pass cover.
• For reservations from Copenhagen to Hamburg, use this page.
• For optional reservations in/to/from/via Germany, use the Deutsche Bahn planner (click on "Seat only”). Often, you can pick the seats. In addition, DB charges 4 EUR per person per journey (max. 8 EUR for a family with children under 15; enter all ages and this will be shown if applicable) and a journey can include multiple trains. Interrail charges 8 EUR per person per train so this can make a big price difference.
• For domestic TGV reservations in France, using Belgian railways (4 EUR fee per order) will be cheaper for you than booking via Interrail (2 EUR fee per seat). The same is true for Thalys and Eurostar reservations.
• Deutsche Bahn can book mandatory domestic reservations for Italy and Spain (not anymore for France), and also TGV France-Spain (often sells out 1 week or more in advance) and Germany-France (often sells out days in advance). All without extra fees. But only at the ticket office or via phone (+49 30 2970). If you pass through Germany, you can order by phone and pick up reservations at a DB ticket machine (using "Bahn-Tix”). They can also send reservations via the post for EUR 5.90.