There are two plugins that do exactly what you want, and I have used them both extensively.
First, ThreeWP-Broadcast. This plugin lets you “broadcast” posts from your sub blogs to your main blog (or the other way around). It also allows you to link to broadcasted post to the original, as well as auto create tags, categories, custom fields, and featured images.
Second, is Multi-Site User Management. This is a little plugin that you drop in your mu-plugins folder and it will keep your users synchronized across all sites, allowing them to login into all the sites with the same name.