wordpress multisite中的用户博客订阅详细信息 - 数据库级别

I want to know where the users blog subscription details are stored in the word press multi-site. If I try to execute the code

is_user_member_of_blog($cookie_user->ID, $blog_id)

it is returning as false but the same check is done in other way

$user = get_user_by('login', $_SERVER['HTTP_SM_USER']);

it returns true.

I understand that user exists in the DB and hence the 2nd statement is true. But what are the basic settings for any user to subscribe to any blog in multi-site.