I see that you are French. If you call SNCF, or go to an SNCF office, they should be able to help you, at least with the ticket from Paris. 

If you are more than 5 persons travelling together you are considered a group and at least some railway companies, like DB, only let you book a maximum of 5 persons together online.

This might be something for you.

https://www.bahn.com/en/offers/group-saver-fare-europe

Thank you for your answer. At the SNCF office, they can’t offer us this train. I call SNCF and I stayed on the phone for 45 minutes. At the end, the officer told me to wait and then hung up the phone without answering.

Why couldn’t the SNCF office book the train?

Hopefully you can get better help from DB.

https://www.bahn.com/en/contact

Thank you AnnaB

SNCF could not answer us. 

I’ve also send a mail to DBahn but I have no answer

on ICE IN Allemagne there is NO need to REServe-but its optional. You will struggle to find empty seats (nonreserved-seats are marked on small screens) for such a group together,

You did not clearly state if you want to reach Wr right away or do some stops to see places en-route. IF yes: go into a DB office and have them arrange it. BUT first double check (and use NOT the app, but the planner of bahn.com itself) to check route and if it is even possible to REServe: f.e. the route via Leipzig-Dresden-Zgorzelec is only local trains without even RESwerv possible (just like TER)

DO find the page on REServ. by seewulf and read it-you likely need it for onward.

Here is the reservation guide mentioned above. 

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

But, as I wrote earlier I doubt that you can make a reservation for 7 people at the same time on-line for all rail companies. At least when I looked at the DB online booking there was a maximum of 5 travellers.

Hello @Jean-Luc,

the simple answer to your question is:

You can't book this train because it has no timetable yet. It should not be possible to book it at all on any sales channel. This is because of track engineering in Kaiserslautern area.

Should it be possible to book to Saarbrücken, be aware:

The actual planning is that it will not running via Saarbrücken at all. But it's not finalised.

Only possible way is to wait (but expect a different timetable and that the train may run via Strasbourg) or choose an other train.

Kind regards, Hektor. 

In addition to Hektor's remarks, SNCF can indeed only book 5 people at the same time online. So once the timetable is fixed, go back to SNCF (ticket office or by phone) to book it.

Thank you for all your answers.

To AnnaB and to rvdborgt:
I was able to get reservations for 7 people through the Interail website for our return trip:
Berlin Frankfurt (ICE) and Frankfurt Paris (TGV)

To Hektor
I understand better indeed. That’s why SNCF only offers this ICE until Saarbrücken

Thank you very much for that answer.  I just have to look for another train. 
By leaving an hour earlier from Paris to Berlin by Koeln, I will find my way back.