Sign up and get registrat on subdomain?

Hi

On my main site i have a link which directes to a subdomain. It is a forum.
On my main site i have a free membership plan which is required to be able to be a member of the forum.
When signing up the visitor also gets signed up as a user on the site.

My question is, what is the pratice here so when visitor signs up one place he/she gets also registered as a user on the other site/subdomain??