If the SFO-DEN flight is on a separate ticket from the DUS-SFO one and is non-changeable/non-refundable, UA will be under no obligation to get your friends to their destination if they don't turn up for check-in at SFO in time and they could wash their hands completely of the matter (to make things worse, they might even cancel reservations for the return flight in case of no-show on the outbound segment).

Many reputable airlines will often accept passengers in that situation on later flights on a stand-by basis, but they are under no obligation to do so and you have no guarantee.

[This message has been edited by NickB (edited 06-26-2000).]