Such simple features? How would you go about doing this?
Some considerations off the top of my head.
1. Where do you store the payment instrument?
2. How do you mark a charging session as completed by someone else?
3. How would you go about contesting incorrectly assigned charges?
4. Who decides who pays and how? If I'm the owner and always get to decide why not just pick the other person when I'm feeling cheap?
Regardless of how simple (or not, in this case) this feature is. You have to think about how often it would be used, which seems like not often. Venmoing someone the $50 in charging they might rack up seems pretty straightforward.