Personally, I'm very cautious on clicking the links in the emails because of security reasons (phishing, etc.).
Unless your goal is not purely to lure the user into your website, I would present the content in the email directly (technically it can be a dynamically generated image, hot-linked image, HTML5) and making the content itself the link to some more information.
To sum it up:
- your app (where the user needs to register) has the option "Share with friend"
- upon clicking you can chose friend's email from your contacts
- an email is sent from a central server and you hope it doesn't end in the recipient's spam box
- the recipients receives a neat email titled "AsafBO shared the weather with you" and the picture and "...more weather..." click prompt.
In such case the link would not diminish my cautiousness on clicking the links in the emails, but I would value the service for sending some information anyway and not being "click or you get nothing" type.