Add user to all blogs and give role


i'm looking for a smart way to add the same users with the same roles in every (existing and future) blog.

So let's say WPMU is a team blog installation and on every blog that get's setup i want to have user "tom" to be added as an administrator, "sarah" to be added an editor, and "mario" to be added as an author.

Any chance to do this with a plugin that already exists? Does anyone know an easy way?