Subdomain Problem w/ login

I’m running a blog that has multiple authors and I’ve set up subdomains for the authors which will show info etc. about each author, but when you go to the subdomain it disregards user info.

Each subdomain’s directory is in the root directory. I copied the index.php file from the root directory (where I have WordPress installed) and changed the ‘require’ code to

require('../wp-blog-header.php');
$_GET['page_id']=XXX;

where xxx is the page_id number. This has worked fine and wordpress seems to be working fine for each subdomain, except for the fact that you can log in from the main domain, go straight to a subdomain where it will tell you you’re not logged in, and go back to the main site where it will say “Welcome! You are logged in as …”

Does anybody know how I can fix this so that the subdomains retain login info, cookies or sessions or whatever?