As per Cougar, if I pay the bill the o/h leaves a tip and vice versa. That said, I’ve worked in the industry for over a decade now and many of my friends have at some point so when we go out we add the tip then just split the bill however many ways necessary.
It is annoying when groups want to split the bill so they each pay for what they had – Can’t possibly risk paying 5p towards a friend’s dinner! Never known people be as tight as they are here in North Wales.
As far as scenario 1 is concerned, you treated family, you pay the tip. Scenario 2, you pass the bill tray round the table and tell everyone to chuck some change in for the tip.