Hey Carolyn,
The easiest thing to do would be to use the
Marketo Forward to a Friend Token. If someone clicks the link they can fill in their "friend's" info. Only problem is, it gets annoying if the partner wants it to go to more than 1 or 2 people.
When somone forwards your Marketo email from their inbox it can mess with the tracking of that lead's engagement in reports.
If you know you have partners that want to send it out again, I would coordinate with them directly. I recommend giving them the HTML of your email except with a special parameter on all links [something like "partner=companyname"]. The parameter value can then be captured on a form field or page view with the Querystring contraint.