Below is the message I receive. So perhaps group bookings can only be purchased by telephone? This seems problematic for multiple journeys. It’s a shame there is no group discount for the Swiss Travel Pass.
“The search results show all the connections available on this route. Online bookings are only possible for connections that do not require prior authorisation and have sufficient space available. Connections requiring authorisation as well as other connections can be booked via the SBB Contact Center (tel. 0848 44 66 88, CHF 0.08/min.) or at an SBB Travel Centre. A seat reservation is not guaranteed. Please note the deadline for advance booking.”