Hi, I would advise you to use bahn.com to do it. You will pay less 4,50€/person instead of 6€+2€/seat of interrail.eu and you can select your seat from a seat map. 

Please note that via bahn.com you can reserve online only up to 6 people. You could split up the reservation or try directly via customer service for Groups of Deutsche Bahn Fernverkehr https://www.db-gruppen.de/contact. 

With seat selection, it's not that difficult to split up the booking and place both groups next to each other.

I gave that a try and for every service I try to book I get this.

That probably means reservation isn't open yet. Just try again later.

Yes it's not possible right now anywhere.

Maybe due to the engineering works around Berlin.

Wait a few days/weeks and then try again via bahn.de. Don't worry it won't sell out instantly anyway

From 8 till 16 July there will be construction works between Wierden and Hengelo. Therefore this train wil only run from Bad Bentheim to Berlin. When you want to travel via this route you will have to use a domestic train to Deventer, change there for a replacement bus to Bad Bentheim and from there on you can get IC 143 (which is diverted between Rheine and Osnabrück via Münster due to other construction works).

I think it's a better idea to travel via Arnhem by ICE:

ICE 225 Amsterdam Centraal 8:08 x Duisburg Hbf 10:04
ICE 759 Duisburg Hbf 10:30 x Berlin Hbf 14:48

or

ICE 123 Amsterdam Centraal 10:38 x Duisburg Hbf 12:32
ICE 549 Duisburg Hbf 13:10 x Berlin Hbf 17:15