If you navigate to your group through groups.google.com and do not see any previous emails in the message list, you will need to turn on Archive options.
Note: To do this you must be an administrator or owner of the group.
To do this go to the group in question, click Mange at the top right.
On the left Go to Information --> Content control and check the box next to Archive Messages to the group:
Once this setting is saved all future messages will begin to post to the forum page for future reference.