When you join a group, you have the option to subscribe to e-mail notifications. In your e-mail settings, you can choose a default subscription for groups you might join in the future.

You can also update settings for groups of which you’re already a member under Your Groups. 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 Changes.

You can also update your notifications for individual groups on the Groups Directory.

You also have the option to change your notifications on a group page.

These are covered in more detail in the guide to group notifications.