It is possible for an administrator of a workgroup to 'push' users of a department or another workgroup into the workgroup.
In order to do this, the administrator needs to have the role of 'Admin af gruppemedlemskaber'.
In the backend this role is assigned via 'System' -> 'Brugere'. Find the category in which the user is imported from, fx 'AD'.
Check the field 'Admin af gruppemedlemskaber' as seen below:
In the frontend, click 'Medlemmer' and then 'Tilmelds arbejdsgrupper og afdelinger'.
In the menu that appears on the screen, select the tabs 'Afdelinger' or 'Grupper' and select the ones you want to add to the group.
This forces all members of a given department or workgroup to become members of the workgroup.
It's possible to see a list of all members of the group in the 'Medlemmer' tab.
To see all the departments that have been forced to be members, click the 'Afdelinger' tab.