Peter Frimberley ) gurgled happily,
sounding much like they were saying :

If it's PAYG, what's the *problem* with several people travelling on
the same card...? Isn't it absolutely the same thing as one person
paying in cash for three tickets for themselves and the two people
they're travelling with?

Well it mucks up the prepay capping, or at least makes the calculation
on whether a selection of journies have qualified for the cap that
much more difficult. Not impossible, but definitely adds an extra
degree of complexity.

True.

Of course two people using the same Oyster card at different time
periods of the day also mucks up the prepay cap,

Which, AIUI, is legit...

but no more so than people sharing a paper one day travel card.

Which, AIUI, isn't legit...

Also what happens if two people get on the tube/bus together, but want
to get off at different stations/stops. The card can only go with one
of them.

Tube would be an issue for that - because of the touching out. Not a
problem on the bus, or if both/all get off together, as is likely to
happen.