Did you ensure that Who can message this group? is set to Everyone for the group you wish to message? And that you clicked “Save Changes”? I just tested it on try.discourse.org and can confirm that it is possible to message the staff group.
Did you ensure that Who can message this group? is set to Everyone for the group you wish to message? And that you clicked “Save Changes”? I just tested it on try.discourse.org and can confirm that it is possible to message the staff group.