It is my understanding, that for the Ad sharing plugin to work, bloggers must have their own Ad Sense account.
According to Google:
How does someone that doesn't have their own site, but want to participate in Ad Sharing, sign up for AdSense, if they do not "own" the site?
To use AdSense monetization products on your website, you must have access to edit the HTML source code of the web pages where you'd like the ads to be displayed. If you submit a site that you don't own (for example, http://www.google.com), you won't be able to place the AdSense code on the site and your application will not be approved.
How will they be able to register using my site, as their Adsense site?