Here is the email I just sent to the organisation I volunteer for. Feel free to adapt and use in your context.
Dear [organisation leaders],
Much of the tech industry (e.g. ) is warning against the use of WhatsApp due to its policy of collecting and sharing user information with third parties and the poor track record of its parent company (Facebook) on ethical issues (see examples  and , and many more).
The situation was made considerably worse with a recent change to the WhatsApp terms and conditions .
So, as your IT person I recommend not using WhatsApp for our work.
We already have an alternative available, and I would be really happy to help anyone who needs help setting it up.