As far as i know, portal is a system to provide access for our external users, ex: customer & supplier. It let you show only a part of your OpenERP application without exposing all features of your system. It is a useful module to allow customers, partners, suppliers and other type of external users to use your system for particular tasks.

Share group is for internal users who have share access, this is what written in OpenERP "Members of this groups have access to sharing wizard, which allow them to invite external users to view or edit some of their documents". You can find share function in "More" dropdown.

The use of Portal under settings is to automatically register the user to portal group if the box was ticked.