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.