2 options I see:
1) Use hooks on the register.php page like do_action( ‘bp_after_registration_submit_buttons’ ) add_action('bp_after_registration_submit_buttons','your_function');
function your_function(){
echo do_shortcode( '' );
}
2) Modify register page –> copy wp-content/plugins/buddypress/bp-templates/bp-legacy/members/register.php to wp-content/themes/your-child-theme/buddypress/members/register.php and modify in your-child-theme
