While Microsoft is working on adding the option to Include Office 365 Group Calendar in Teams, you can use this as a workaround which will basically add a web link to the group calendar as a tab in Microsoft Teams.
In the left navigation menu, find your Team under the Groups section and click on it
In the right pane, click on the Calendar menu (click on the image above for reference)
Copy the URL of the calendar (which should look similar to https://outlook.office365.com/calendar/group/tecklyfe.com/tecklyfetestteam/view/month)
Open Microsoft Teams and go to your Team
Click on the + to add a new tab and select Website
Name the tab and paste the calendar URL into the URL field and click Save
You might need to sign in with your Office 365 account the first time you navigate to the new calendar tab in Teams. You’ll also notice that it’ll overlay your personal Office 365 calendar with the Team calendar. You can uncheck any calendars that you don’t want to see, but it might change back to the defaults the next time you go to the calendar tab in Teams.