Google Groups is a tool for a team of users to quickly collaborate with one another either via email, or any of Google's various applications. With members of your team all in the same group, Google Groups makes it easy for sharing things like Google Calendars and Docs with one another. Documents and things shared with the group are now available to the individual members of the group.
Lose a team member? Although unfortunate, it remains important to remove access to potentially confidential information from those who no longer have permissions to view it. With Groups making it easy to share documents, once an account is removed from the group all of the documents that were shared with the group are no longer viewable from that account.