Quick comparison of the chat apps
Comparison of the drawbacks and benefits of Mattermost, WhatsApp, Telegram, and Signal. For more detail see Choosing a messaging app for your XR group in the "Online Tools for Communicating with Other Rebels" book.
Here is the summary from that chapter:
Ultimately which tool you decide to go with comes down to what you and the other people in your group are comfortable with. However, our recommendations can be summarised as:
If your group...
...organises lots of different things at once Use Mattermost. The extra features and discoverability should really help to make your group more productive.
...organises a lot of actions Use Mattermost combined with Signal for the added security. Also don't forget to enable disappearing messages.