¶ 
Matrix - status quo on security
There are plenty of solutions like Rocket Chat, Slack or Mattermost. But guess what the German Military uses because of its superior reliability? It's Matrix.
At Libcom.de we have been using Matrix since 2018 so we know how to handle this excellent piece of software. To be honest, the software used isn't called matrix but it's synapse and we can run it for you. No matter if you want it on premise or if you want it to host it for you. Libcom.de is the way to go.
¶ Features
Matrix uses several methods to make sure only authorized people can see the texts. There are of course groups channels that can hold thousands of users. Next there is one to one conversations.
You can get bots that answer questions and integrate it into your monitoring solutions, so you won't need no more pushover or similar services.
Matrix can be run in a clustered way so it scales even for large usergroups.