I understand, you have your own members implementation.
You can add extra information before saving using a “before save” hook, so you can do something like this:
$w("#myDataset").onBeforeSave(() => {
$w("#myDataset").setFieldValue('emailField', userEmail);
});
This will set the field in your collection to whatever value you give it, and it will be saved as part of the current record being saved.