If you want to assign an announcement to portals, you can do so by clicking on the Portals icon under the Action column on the Manage Announcements page. You will be redirected to the portals listing page of the announcement. On the users listing page, press the plus button at the top.
You can select portals from the list. Press ASSIGN button to assign announcements to the portals. This will make the announcement available to all the Super Admins of the selected portals.