To set up a mailbox that forwards emails from a Google Group, you'll want to select the No, it's a group or alias option when you first add your mailbox. 

When you select this option you'll be given an forwarding address. 

Next you'll want to head to your Google Admin dashboard and click on Groups.

Then simply edit your group and add your address as a member. 

After you've added your new member, send a test email to your group email address and it should arrive in Groove shortly.