Post some posts to main blog

Building a site and trying out multisite to see if it fits my purposes. The site will only have 4 predefined subdomains. I want to let the users of a subdomain have the option when they publish a post to also publish it on the top domain. Is there some reletively easy way of doing this? Alternatively, cmaybe at least all posts form the subdomain cna be automatically posted to the top domain.