This requires administrator rights in Pronestor Planner
The users can download an installationfile to install Pronestor Planners Outlook add-in on a local installation of Outlook under "my settings" in Pronestor.
But first the Pronestor administrator has to enable the installationfiles.
Go to "Settings"
Click "Other settings"
Scroll down to "Outlook installers"
Here you can create the installer that matches your users. There is a difference between a 64-bit installer and a 32-bit installer. The "Outlook Add-In 2016/Office365 for Pronestor" installer is a 32-bit, and so are all installers without 64-bit in the name.
You might need to enable multiple installers if your users use different versions of Outlook.