If you use ArcaneChat (https://arcanechat.me/) or DeltaChat (https://delta.chat/) there is an in-chat mini-apps for split bills, you can use it with groups of friends totally private end-to-end encrypted between chat members without the need of any online service for split bills
It is for encrypted email only, its possible if the other people use some client that supports encryption, if you question is “can the email provider be used to send and receive unsecure unencrypted email that the server would have access to and read all my conversations?” the answer is no, the app can be used with such unsecure providers but arcanechat.me server is focused on security and no unencrypted email/data is ever stored on the server