Hirers' own details
Hirers keep their own details up to date in their app: under Your account, Change my details lets them change their phone number, address, the group’s contact person, and a second email address for invoices (a group’s treasurer, or a company’s accounts address). Each change is noted in the Audit Log.
Their name and the email they sign in with stay with the hall: the name is who the hall agreed the hire with, and the email is how they sign in. To change those, they contact you and you change them on the Hirers screen.
Invoices go to the hirer’s email and, when there is one, the second address too. A phone number that is not a UK mobile cannot take text reminders; the app says so.
For example: The Brownies get a new treasurer. Their leader adds treasurer@example.org as the second invoice address in the app, and the next termly bill goes to both of them.