subdomains and easysocial

I have a site that will have school A and school B on it. You can enter the site thur a subdomain to get to school B. My question is how can I make it so that whether you enter the main site or the subdomain site, your easysocial login is the same and the users on either site(subdomain) will all be apart of the same easysocial?

I guess, also I dont understand how subdomains truly work either?