Display site administrator’s id by current blog id inside link

If you want to construct for example:

http://example.com/?ref=1,8,22

for multiple administrators of the current sub site, you can try the following (untested):

$uids = get_users(
    array(
        'blog_id'  => get_current_blog_id(),
        'role'     => 'administrator',
        'fields'   => 'ID',
        'order_by' => 'ID',
        'order'    => 'ASC',
    )
);

echo $url = add_query_arg( 
    array( 'ref' => join( ',', $uids ) ), 
    'http://example.com' 
);