To use the JOIN Outlook add-in you need the O365 module in your JOIN license. If the module is available, you can download the manifest for the add-in from the page /add-in/Outlook.xml under your JOIN site, for example https://myorganization-jzd.decosasp.com/add-in/Outlook.xml. After entering this URL, a file called JOINOutlookAddInManifest.xml will be automatically downloaded.
To test the functionality of the add-in, you can add it for one user. Microsoft calls this sideloading.
+ Add Custom Add-in at the bottom of the page.A few minutes after installation, the add-in will become visible in both the Outlook desktop version and the online version. You may need to restart the desktop version.
You can add the JOIN add-in as a custom app in the Microsoft 365 admin center and then assign it to a group of users within your organization. For this you need the file JOINOutlookAddInManifest.xml for your JOIN website.
More information can be found on this Microsoft page:
Recommended approach for deploying Office Add-ins