Pro Sites question - How to create multi-layered structure

I'm being asked to set up a site similar to https://edublogs.org/, where we can create an organisation, and an admin in that organisation can create multiple individual blogs for each of the members of that organisation.

You say that pro sites was used for edublogs? I can't see from any of the documentation how you create that multi-layered structure? Are there any instructions anywhere?

Thanks for your help.

Jake.