The guidance says,
"Employees you can claim for
Furloughed employees must have been on your PAYE payroll on 28 February 2020, and can be on any type of contract, including:
full-time employees
part-time employees
employees on agency contracts
employees on flexible or zero-hour contracts
The scheme also covers employees who were made redundant since 28 February 2020, if they are rehired by their employer."
https://www.gov.uk/guidance/claim-for-wage-costs-through-the-coronavirus-job-retention-scheme
Folk can be re-hired and be part of the scheme but it is up to the boss whether that happens or no.