WhatsApp is a hugely popular messaging app with over 2 billion users. It's easy to use and the vast majority of rebels will already have it installed on their phones. Does this make it a good fit for your XR group?
Security
Despite being end-to-end encrypted, WhatsApp has a number of serious security issues that make it a poor choice for serious organisation in XR. To start with, it is owned by Facebook, a company whose income depends on collecting people's personal information. They can't read your actual messages, but there's still lots of metadata they can collect. You can read WhatsApp's privacy policy to get an idea of the sorts of information that they are collecting.
Another issue with WhatsApp is that it makes it easy to backup chats to a cloud storage (like iCloud or Google Drive), and that by default the option to make these backups encrypted is off by default. That means that any user in your chat who has a backup running (which is common) and hasn't manually set this to be encrypted will be uploading the whole chat (their messages and everybody else's messages in the shared chat) to a server. The data on that server can be read by the company (or security services with a warrant).
Accessibility
WhatsApp has a lot in common with other messaging apps so it is usually quite straightforward for rebels to learn how to use it. However, in order to sign up to WhatsApp you need a smartphone, which not all rebels will have access to. In order to use WhatsApp on a computer, the smartphone that it's linked to must be connected to the internet continuously, which is a hindrance.
Features
You can't save messages for later or mark them as unread so you constantly lose key info and can't find it again. The message box is small and you can't thread effectively so it is difficult to keep track of a conversation.
Discoverability
Within WhatsApp, group admins can create and share links that rebels can follow to sign up to the group. This provides a quick and easy way to invite new people. However, unless this link is published somewhere for rebels to find, it is impossible to find the group to join in the conversation.
Other information
Another issue with WhatsApp is that many people use it to speak to friends and family. Having XR-related chats on the same platform can put unnecessary stress on rebels who might want to temporarily 'switch off' XR communications in order to avoid burnout.