Choose Share on an existing report, enter the person's email, choose + Add, select Reviewer, then choose Save Changes. You can also select a Reviewer on the final Create a Report screen before creating a new report.
The Reviewer receives a secure email link to a focused review page. They can inspect the report and receipts, edit the shared Google Sheet, comment, approve, or request changes. If they request changes, fix the report and choose Send back to reviewer.
Reviewer access is report-specific. Removing a Reviewer from one report does not change your Default report sharing list in Settings. Removing a Reviewer from Settings removes them from future reports but does not alter reports already shared with them.