Paymates' developer takes your privacy very seriously. Other than data you enter yourself, Paymates does not collect any kind of data simply because this is a hobby project and setting up a data pipeline is non-trivial.
Since you can choose to share expense and group information with others, that data is stored and shared in your personal iCloud account and will be viewable to those you share a group with. Still, that data is stored on your own iCloud account, not accessible by me or even Apple.
At this time, Paymates does not collect anonymized information about usage and performance, and includes no third-party services that may do this on behalf of Paymates. While this could change in the future to help analyze Paymates' behavior out in the wild, it will always be done with your privacy in mind and only with anonymized data unless you give permission to include actual data.
If you have any further questions or concerns, please reach out to support@getpaymates.app.