Calling SNCF tomorrow is your best bet :

- French railways (SNCF) ticket office or by phone (press #85 for English, no booking fee, delivery via e-mail)

They should be able to help you, although they somehow don't accept all credit cards (but British ones should work)

I called today, 90 days before my trip.  The very helpful lady could not book reservations for a group, and gave me the group booking number which only seems to be in French and had closed for the day.

Well she could have split the booking in 2 or 3 groups : one booking per 6-people couchette compartment for example. I don't know why she hasn't offered you that option... maybe try again tomorrow with someone else or maybe the group booking number would help