Of course you'd want any tweak to increase revenue. Allowing strangers to do on the spot group purchases would effectively lower prices during non-peak times, conceivably increasing revenue through more sales. And I suspect due to coordination problems most group purchases would not be made with true strangers. However it seems that even free transit does not increase ridership much (one data point -- "spare the air" free days only bring in 8-10% more riders) so I suppose one has to be extremely careful with discounts.

Caltrain would be a little easier than Muni Metro because the former already has the concept of multi-ticket purchases in their vending machines. Muni is so slow and uncouth I'd rather walk, but that's neither here nor there.