Signal or XMPP are good option

XMPP may be a bit overwhelming as you need to select a provider and a client. Doesn't require phone number.

Signal is private and easy to use like whatsapp. Needs phone number to use.

Telegram is cool but encryption isnt enabled by default, would like to avoid it for this. Cool for public groups.

Edit: Teleram isnt good at all, dont use it. Look reason in the replies