Updated 07/16/14 JE
Annotations are visible to any user who can view the source profile page in AIR. It is not limited to users that share the organization of the annotator.
Annotations are never visible to sources via the Source app.
1. Go to the source's profile page and click on the expand button in the upper right corner of the Annotations panel
2. Click on the "new" button
3. Enter the annotation and click save
4. You're done! You can also click the pencil icon to edit an annotation or the red X to delete it. You can only delete your own annotations.