Is there a way to add users automatically to certain blogs?

Hi All,

Does anyone know if there is a plugin which would allow me to add new users to certain blogs.

Let me set the scene for you.

I have a multisite install with the main blog, a sub site acting as a directory, a sub site as a classifieds section and a sub site as a shop.

So if a user registers for any one of these site I want them then to be registered for all the other 3. Is this possible?