Gathermate
Two places are already added. Enter each bill, who was there and who paid.
Six for dinner, four for drinks, two for the last bar? Each bill is split only between the people at that place, then added up per person. Rounding happens once per person, so you collect less extra than rounding each bill.
If one friend paid for dinner and another paid for drinks, pick who paid each bill. Gathermate works out who should send how much to whom, with as few transfers as possible. If nobody is picked, everyone pays the organizer as usual.
Six people have a $300 dinner (Alex pays), then four of them have $120 of drinks (Sam pays). The four who stayed owe $50 + $30 = $80 each and the two who left owe $50. Alex gets back $220 and Sam gets back $40.
Yes. Give each group a weight, for example 1 for most people and 1.5 for someone who ordered more, and each share is worked out from the weights.
Choose to round each share to the nearest 1, 5 or 10 and whether to round up or down. You see the extra or the shortfall straight away.
Yes. Everyone can mark themselves as paid on the shared page, so the organizer can see who still owes. Because that is self-reported, the organizer can also mark money as received once it actually arrives.