By default, any content students create in Mahara, including their journals, is private to them.  This is a feature built in to the platform that is actually reinforcing the desired model of having students make considered choices of what they publish and share.
The idea is that students select the artifacts that best represent their learning for a particular purpose, whether this is a class, activity, tutor group, or parent audience.

This video demonstrates how to simply share an entire Journal by creating and sharing a page in Mahara.