There are several ways for students to share their Mahara portfolios with you. This can be done from within Mahara by having students look for you in Mahara and share their page with you. Instructions on how to do this are available here.
However, the easiest way to share, view and grade portfolios is to use the Mahara assignment option in Moodle. This option is based on the familiar assignment activity and gives you the option to set various parameters such as the possibility for students to keep working on their portfolio once they make them available to you. The video below describes how to create a Moodle assignment and set all the related options.