There is a Signature field located directly below the field where you type your reply to a member comment.
This is where you have to option to add a signature to your message. You can also include your contact information if you would like the member to have the option to email you back.
When responding to a member comment, you also have the option of clicking Apply to all, Apply to just this reply, or Remove signature.
You can change these settings at any time by checking or un-checking the radio buttons while responding to a member comment.