As of Collect v1.25, it is possible to track both user identity and change reasons in the audit log.
XLSForm support is not yet available. In the mean time, the identify-user spec and track-changes-reasons spec are available. To add these options to your XLSForms, you can add columns bind::odk:identify-user and put true in the audit row and/or bind::odk:track-changes-reasons and put on-form-edit in the audit row.
There is also a test form on the default server to try out the user interaction.