Pages can be shared using any means available under the Salesforce Security Model. Depending on your situation, it might be easiest to use Sharing Rules to share pages with the Roles / Groups associated with your external portal users. The other option is to manually share individual Pages with certain Users — this can be done by going to the Page Composer and clicking the “Share” button. This takes you to the Sharing page for the Page, where you can add as many Manual Sharing entries as you want to share the page with internal or external users.