This has nothing to do with Eurail/Interrail, so I’m not really sure we’ll be able to help. I don’t think anyone will make a point that there’s one passenger missing though.

Hi Schelte,

Apologies, I did not realise it. But thank you for the advice :) 

No worries, it’s just that I’d be surprised if someone else here had this issue with SNCF group reservations. But who knows.

I may just give you the official rule: Si vous modifiez votre voyage, votre groupe doit toujours contenir un minimum de 10 personnes et un maximum de 36 personnes. 
Si ce n’est pas le cas, annulez votre réservation initiale.

So at least officially it seems that it isn't allowed (according to the last sentence) if you do know it yet that there will be less persons. But it's always possible that someone won't show up at the day of departure. Don't know what's to do then or how they will handle this. 

I wonder if the rule is if you change your train (trip) you still need a minimum of 10 , but if you simply turn up with 9 what are they going to say or do?

Obviously you cannot cancel a single person as it is a single group reservation but you have paid for 10 and only 9 will travel, but if you were to modify the reservation you would still need to have a minimum of 10.

I was wondering this also. I have queried with the train company on this. But I appreciate the advice. I will let you know if I get any response!

SNCF is an utter state-run burocracy that lives to the rules it has been set to from some MInistery, so that same answer you will get-If you are so lucky that there is someone who is able to handle non-FR questions at all. The same as hektor has cited above.

For TGV every pax needs to have her/his own ticket with barcode for most stops to pass gates. Now very intelligent systems could detect that there is 1 groupticket/member too few for the 10 needed, but I highly doubt if it is so advanced. On my last trips (not that many- 3 or 4) in ´22 there was never a check IN the train. Just tell ´so sorry, A fell ill this morning¨ in the unlikely case you are challenged.

Perhaps a groupe de dix=10 is even cheaper as booking for neuf=9?

I have heard back from SNCF. In the scenario when only 9 people travel in a 10 person group booking (travel for 9, paying for 10), the group is still allowed to travel. If 8 or less people travel the group is required to cancel their ticket. I hope this will be useful for someone in the future!

Always good to get some official feedback. I would never have thought they would have such a rule.

Thanks.