Allow a bill to send email notification to all involved
I see that you don't want to spam by enabling email notifications by default, but what if you allow the bill creator to check a box saying "notify all involved in this bill," and when you send the email, say "User X asked us to send this to you" (or something to that effect).
This a decent suggestion – I’ll pass it on, though I’m not sure it’ll get on the roadmap in the near future. We tend to err on the side of caution, because when we send emails without users’ permission, we often get flagged as spam or have to deal with angry replies :( But this makes sense, and would definitely be useful.
Amal Chandran commented
Or you can have it as some kind of notification within the splitwise webpage/apps