Share this TO wordpress site?

I’m running a wordpress multisite. Does anyone know of a plugin or a way to create a “Share this on XXXXXX.com” the same way that Facebook and Twitter etc do?

A lot of my members have blogs outside of my network and have to do double posts. One to each space.

So, is there a way to create a button and a link that they can add to their other web spaces that they just need to click to automatically post under their account on my network?

Think of it as a reverse social sharing button, or simply adding my site as an option for them to share their external content to.

Thanks for any advice!