Setting up a multi-user site

Ok, here I go. I want to set up a new multi-user site for an organization. They want to offer the people that work with them the possibility to easily start and maintain a blog. Plus the option to have a homepage where those blogs are shown. A bit like the edublogs, really. So, an automated process to start a new blog, choosing templates, widgets etc. However, registering a new blog is only open to people who work with them, so it should have some sort of restriction like having to fill in a code that they will be given by the organization when they start a new blog.

I have built Wordpress blogs before and I know my way around them. I already set up a multi-user Wordpress site, but:
- what are the plugins I need to make this happen
- how can I make a certain widget mandatory for all blogs for instance? Or advertise across all blogs?

Thanks!