When you join a group, you automatically receive e-mail notifications for all group activity. If you’d like to change your notification settings for one of your groups, please go to the home page of that group and look for E-mail Options in the left navigation panel. On the next page, select your preferred option: No E-mail, Weekly Summary E-mail, Daily Digest E-mail, New Topics E-mail, All E-mail. Then, select Save Settings.
These are covered in more detail in this guide.