As a member of a Google Group, you can control a number of settings, including how you'd like to receive messages from that Group. Google allows users to choose from:
a single daily summary of all messages, delivered at an unpredictable time of day
a batch, or "digest", of up to 25 messages per day, delivered at an unpredictable time of day
an individual email for every message, delivered as soon as it was posted/approved
This document walks users through managing these per-Group member settings.