Main multisite (portal) with content from undersites?

I have a multisite with 5 sites under it. The 5 sites are controlled by different bloggers.

Now i want my mainsite to show most recent, read and debated posts, and maybe the most active blogger (dont know if thats possible). So my mainsite will be like a portal of all the other sites.

My question is how is, is this possible and how is it done? Im not asking for step-by-step walkthrough, but if anyone knows about a tutorial or article about this subject or can point me in the right direction, any help would be very much appreciated.

/jaze