If you do not see Announcement action in Chatter Group, check following items:
1. Enable Publisher Actions
Setup | Customize | Chatter | Settings, make sure Enable Publisher Actions is enabled.
2. Add Announcement in Group Layouts
Setup | Customize | Chatter | Groups | Group Layouts
Click override the global publisher layout link, you should see Announcement there and you can move it to the left if needed. You can revert to the global publisher layout by click arrow icon at far right in Publisher Actions.
In this setup menu, you can create new Group Page Layout and assign it to different user profiles.
Only following users can post and delete group announcements:
- Group owners, or
- Group managers, or
- Users with the “Modify All Data” permission
Post Announcements in Chatter Groups