Do you mean replicate what the Forward button does in an email client? If so, I don’t think that is possible.
I’ve never seen a share button in an email, as they usually require JavaScript. You could use this http://www.sharelinkgenerator.com/ and link to the hosted version of your email - that should work pretty well.
To get a link to the hosted version of your email, send it to yourself, view the source (easiest to do in Gmail) and grab the hosted URL.
Has this been answered? Or have you figured out how to do this? We too would like to add social share buttons to our newsletter (not just links to our social accounts). The forward button would be nice too!
We now have to add a Text Block then this merge field code, ~HostedEmail.Link~, to get that link to display on an email. See before and after screen shots.