The comments library is inside SpeedGrader. You can find it by clicking on the small speech bubble just above the comment box in the bar on the right. Type a comment in the Add comments to library box and click the button below the box to save it.
A tip is to save comments that function more as templates than as completely finished comments.
To use a saved comment from the library, click on the small speech bubble and on the comment you want to use. It is then added to the comment box for the student's submission and you can edit it if necessary before clicking Save to send the comment to the student.
If you want, you can activate the Show suggestions to get suggested comments without having to click on the speech bubble.
The comments you have saved are only available to you. It is not possible to share comment libraries with a colleague.