I have possibly done similar things, can you explain a bit more what you have in mind?
In my experience the best think you can do to create a strong community to enable and facilitate. How brave are you? ;)
I'd recommend you look into open-space / unconference approaches to running events - put the grads in the drivers seat, let them set the agenda and let them loose!
Get the graduates talking about what matters to them, what it is like to be a graduate in your organisation, the challenges they face and their ability to support each other and collaborate. Despite differences in placements, backgrounds and career goals - graduates are learning in parallel. The trick is to get them to explore the ways they relate to each others work and the opportunities for mutual support and collaboration.
A graduate event is also a great opportunity to seek feedback on the organisation from "fresh eyes". Recruiting your own graduates in improving the business they have joined can also bond them, but make sure the organisation is willing to listen first!!
Good luck :)