You can communicate through phone to all new hires and then email. It is not clear whether Appointment Order /Offer letter already issued to them. Assuming it is issued, you can present as below.
" With further reference to our Appointment/Offer letter dated______please be informed that due to administrative reasons your date of joining with us is postponed to ______.
Please report at our office on _____
20th May 2019 From India, Madras