I just found this topic while searching for the same problem.
Also you cannot add foreign recipients via the webgui without adding them as contact before (which makes them displayed in everybodies addressbook, if I understand correctrly?)
I just worked through the grommunio-admin mlist and think I found a workaround by simply doing it via cli:
grommunio-admin mlist create mylist@firstdomain.tld -t normal -p all -r user1@firstdomain.tld -r user2@seconddomain.tld -r someone@foreigndomain.tld
the created list looks valid when displayed with
grommunio-admin mlist show
but is broken when viewed in the admin GUI.
However I have not tested yet to send an email to it
It would be great if this would work via GUI!
Regards
Matthias
Edit:
Also I have not found how to activate "Hide user from Adressbook" when using CLI
Thus the workaround changes to Adding a new empty Group in GUI and filling it via CLI.
Also other options like alias are not available in GUI.
In addition, when editing a group via GUI that holds members (like from other domains) that are not visible for the GUI. they get dropped, when editing that group in the GUI.