If someone forgot to sign a message they posted on a talk page, you can do it for them by using the ((unsigned2)) template. Simply place the template at the end of the comments, and include the user's name and, if possible, the timestamp (which you can get from the article history}. For example: ((subst:unsigned|MyNameisForgetful|21:18, 11 January 2016 (UTC))) would look like this:
— Preceding unsigned comment added by MyNameisForgetful (talk • contribs) 21:18, 11 January 2016 (UTC)