The below link helps us to place custom javascript at launch, but here, you need to get the email HTML element class name or id attribute to fetch email address and at the same time, you need to get the share button class name or id attribute to perform client event. based on this you need to write your custom logic.
https://docs.adobe.com/content/help/en/core-services-learn/implementing-in-websites-with-launch/conf...
Alternative way is you can do the same thing in the frontend.
You need to get the logged-in email id in the custom javascript, then on click event of share button build data layer JS object and send it to the _satellite.track("shareButton") method.