Mattermost (MM) is a messaging system. All XRNL activists are expected to be on MM.
Activists can use any username (like @Fred_Amsterdam or @JimForTheClimate), so a certain level of privacy is possible. You do have include an email address when you register, though that is not visible to other users.
On joining you will be made a member of a particular "Channel". Channels are set up for types of activity, particular locations, and for once off events. More on Channels below.
Channels in Mattermost
Most channels are done in English, some where most posts are given in both English and Dutch, and other channels Dutch only. The convention for which language is Still-finding-out.
Channel types with (clickable) examples
For a location team - most locations have a channel called "Town Square" or "Dorpsplein". Examples
Most Channels are public and so readable by all MM users - in practice that implies information is accessible to the whole world. There are activities which require messaging to extend to a small group of people - and that is when a Private channel is appropriate - you may receive an invitation to join a Private channel, which will then come up in your MM Sidebar.
You can also message an individual or a small group (up to eight users) for a conversation which does not fit into the mould as a Channel. Direct Messaging usage works much the same as WhatsApp. Ask your XR colleagues what is their ID on Mattermost.