add_filter('woocommerce_is_purchasable', 'your_custom_function', 10, 2);
function your_custom_function( $purchaseable, $product ) {
	$purchaseable = false;
	$is_a_member = false;
	require_once( ABSPATH . 'wp-includes/pluggable.php' );
	if ( $group = Groups_Group::read_by_name( 'Foobar' ) ) {
		$is_a_member = Groups_User_Group::read( get_current_user_id() , $group->group_id );
	}
	if ( $is_a_member ) {
		$purchaseable = true;
	}

	return $purchaseable;
}