This article explains issues with the workplace from Facebook spoke, and its solution:
Docs link about Workplace from Facebook spoke, and its setup :
As part of Group Management, user addition to groups fails to add users to groups.
While configuring workplace from Facebook spoke, the setup prompts to select whether "specific group" or "All groups".
There is a limitation from the FB workplace that specific group selection will not be honored.
Select "All Groups" instead of 'specific Group' while configuring the FB workplace spoke.