Are you sure the TA gave you correct information?
Normally if somebody out of a group does not show up for check in the rest of the group can still travel. Depending on the kind of ticket the value of the no show ticket is zero or almost zero.

Why don't you ask the TA what would happen in that situation and ask for a specific reason why all members of the group need to check in together?

I assume you are not expecting a refund for the ticket of the no show.