Sign in to the same AmongFolk account you used for the purchase. Make sure your phone is also using the Apple or Google Play account that paid for it.
Open You → Plans & billing and choose Restore App Store purchases or Restore Google Play purchases. Wait for the result, then check your group and, for Together, Your slots. Restoring a purchase does not start another subscription.
If someone else pays for the group, you do not need to restore their purchase. Check that you have opened the right group and ask the payer to check its coverage.
If you have changed between iPhone and Android, purchases are still managed by the original store. Contact support if the plan does not appear on your AmongFolk account. Include the store, group name and purchase date, but never send a password or full payment details.