O365 groups are working properly in Office 365 ProPlus, but are O365 groups also supported in Office 2016 Professional?
Althought there is a node "Groups" in the navigation pane, it remains empty in Outlook from Office 2016 Professional.
Yes, they are supported there as well, I'm using them on both platforms. Make sure your Office 2016 Professional Plus is up-to-date and Outlook is running in cached mode.
Cached mode is on, but I doubt the latest update of Professional is deployed. Any idea of minimum version?
It is odd that users see the Groups node but it isn't populated.
According to the date of publishing this article, it should be supported for long time. But having the latest patches installed is always a good idea if troubleshooting is needed.