Per this UserVoice post, Microsoft is working to make Group Calendars native to Microsoft Teams via the “Meetings”/Calendar view, but below you’ll find one potential workaround in the meantime.  Please note that the below workaround may not work for users with On Premise mailboxes.

1. To add a group calendar to Teams, we need to acquire the Calendar URL first. Log into the Outlook Web App (https://outlook.office365.com/)
2. Go to the Group you want the calendar for:
   
3. Click “Calendar”:
   
4. Copy the URL from your browser:
   
5. Add a new tab within a Channel in Teams:
   
6. Select “Website”:
   
7. Add a name and the URL you copied from the Outlook Web App:
   
8. Sign in to Microsoft when prompted. This tab will now take you to the Group Calendar for the group you’ve selected.  Just be aware that you’ll need to sign in to your own Microsoft Portal account to display the group calendar: