Allowing users to be online on their subdomain

Hello,

I got a standard WordPress installation running, no multisite activated.
I have made a few modifications, one of them is allowing users to create a profile that they can reach from user.mysite.com, similar to your Facebook “Wall” etc.

My problem is that they can login and change their information at mysite.com, but not at user.mysite.com. I want them to be online at user.mysite.com if they login at mysite.com, is that possible? That way they can edit their information directly at their own place.

Best Regards,
Anders