wp_admin_user_asso.php

add_action( 'user_register', 'myplugin_registration_save', 10, 1 );
function myplugin_registration_save( $user_id ) {	
		$current_admin_user = wp_get_current_user();
    if ( isset( $_POST['first_name'] ) ){
		$user = new WP_User( $user_id );
	    $user_roles = $user->roles;
	    $user_role = array_shift($user_roles);
	    if($user_role!="administrator"){
	    	update_user_meta($user_id, 'asso_admin', $current_admin_user->ID);	
	    }else{
	    	update_user_meta($user_id, 'asso_admin', $user_id);	
	    }        
    }
}
 
 
add_action('pre_user_query','adminuser_pre_user_query');
function adminuser_pre_user_query($user_search) {
	if(is_admin()){
	  $user = wp_get_current_user();
	  if($user->ID!=1){
		global $wpdb;
        $user_search->query_from .= "JOIN {$wpdb->usermeta} um ON um.user_id = {$wpdb->users}.ID AND um.meta_key = 'asso_admin'";
        $user_search->query_where = 'WHERE um.meta_value=' . $user->ID;	  	  	
	  }
	}
}