Mattermost is team chat inside your organisation, with a channel for each team, client or project. It works in a web browser, and it has phone and desktop apps.
It comes with the Office solution.
What Clusterz sets up
Mattermost and its database, at its own web address, with an administrator account. It is sized for about 50 people. Clusterz keeps Mattermost's mobile push relay switched off, so message text does not pass through its makers' servers.
To add it on its own, open Mattermost on its own. Install a set or an app gives the steps.
How to sign in
The username is admin, unless you chose another as you installed it. First sign-in to an app gives the steps. You then bring your staff in from inside Mattermost.
Licence
Mattermost is open-source, under the GNU Affero General Public License 3.0, with some parts under the Apache License 2.0. These apply to your use of it.
Learn to use it
Training your staff stays with you. Mattermost's makers publish the Mattermost documentation.