That’s a great idea, it sounds like a combination of this idea -
https://community.monzo.com/t/grouping-notes-on-payments-for-special-events-days/6548/2
& the bill splitting feature that’s on the way -
https://community.monzo.com/t/bill-splitting-1/6397
here’s a preview of version 0 of bill splitting -
https://community.monzo.com/t/split-the-bill-via-monzo-me/7276