You will have to add a new page template, which is a copy of the template that you already use for your pages.
Then simply add a new loop:
$portfolio_items = new WP_Query( array(
'post_type' => 'portfolio',
'posts_per_page' => 5,
// '' => '', # Other arguments here
) );
if ( $portfolio_items->have_posts() )
{
while ( $portfolio_items->have_posts() )
{
$portfolio_items->the_post();
// Example output:
the_thumbnail();
the_content();
}
wp_reset_postdata();
}