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 XXXX@inbox.groovehq.com 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 XXXX@inbox.groovehq.com 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.