For any group you can change the frequency of emails that the group receives from the Group Settings page to one daily email, weekly, or monthly. All members of the group will then receive one email with all the notifications sent during that period of time.
To get to the Group Settings page, navigate to the Groups page and click on the title of the group you want to change:
Click on Settings:
Then scroll down and expand Custom Email Options:
Check the box next to Subscribe all members to an email digest:
When you check the box, you can choose the frequency of when group members receive emails from the dropdown menu:
Then make sure you check the box to use the digest options of your choice!