You can choose to receive email notifications for specific groups as digests, so that you will only receive one email with all the posts sent during the selected time-frame from your profile notification settings.
Go to your profile via the dropdown menu next to your profile picture:
Then click Notifications in the left-hand column of the page and choose how you would like to receive your email notifications from the drop-down menu:
Click Save at the bottom of the screen to save your selections.
NOTE: When a group is set to no unsubscribes, you are forced to receive the emails as they come (used especially for emergency messages).