Hello illinivet
First of all, welcome to the SBB Community. This customer support forum is run by humans, all volunteering travel experts, not by AI.
No, group reservations must be made at least two working days in advance.
If you miss the departure time, you will not benefit from reserved seats and have to take a train that has sufficient space. You should plan enough buffer between the flight arrival and the train departure.
The group reservation has the advantage that you will have reserved seats. The train operators are thankful if groups announce themselves in advance, as it allows the train operators to plan their rolling stock accordingly. And, they grant a discount on tickets for doing so.
Please find all information on group reservation here:
https://www.sbb.ch/en/tickets-offers/tickets/group-tickets-switzerland.html
Kind regards,
Roland