This depends how you share:
Clicking the 'share' under the original post means that the original poster sees the share. Unless it has already been re-shared and you are actually sharing that post. The comments in any shared post will not follow, nor does the originator see your comments.
- No matter with whom you share a post you’ve found in Google+, it is always polite to mention the person who shared the post before you. Simply add ‘Via +Mr x’ or ‘Thanks for the share +Ms Y’. Also it is very nice if you write your opinion (or just a few words why you share) about the post you are re-sharing.
Linking the original post to a new one by clicking on the ‘time stamp’ and then posting the permalink is a good way to credit the original sharer and keep your own commentary intact for possible reshares. The downside is the original writer doesn’t know that the post has been shared, unless you mention them in your new post text. If the comments are really good in the original poster’s thread, sharing the link and then asking people to comment on the original while disabling comments on your re-share is a good practice.
(from my article http://mediatapper.com/google-plus-etiquette-sharing-is-caring/