Let users register on any subset and become users of all sites....

Hi All. My scenario is this.

I am developing a multisite wherein the primary site is a business directory containing many individual listings some of which also maintain a subsite on the network. Their profile page on the directory links to their site.

I want new users to be able to register on the primary directory site and to be automatically "logged in" should they visit any of the hosted subsites.

Additionally I want any new user to be able to register via any subsite. The user would be sent to a registration page on the primary site and then redirected back to the originating sub-site.

What is the best way of accomplishing this?

Many thanks,

Briany Hearne
Dublin, Ireland