You can create a group calendar in Teams by following the steps below.
Select the Teams Channel you want to create a calendar for
Go to General
From the top navigation bar, go to Files, then click on Open in SharePoint
It will take you to the SharePoint site.
Go to Home
Click on New > App
It will take you to a page where you will be able to choose from the apps, choose Calendar.
Choose a name for the Calendar app, then click Create.
It will take you to Site Contents, open Calendar.
Once you open Calendar, copy the URL of the page.
Now, go back to Teams app.
Click on the '+' icon.
It will take you to a page where you can select a Tab.
Now, In the new tab add a Tab Name, then enter the URL you copied from the SharePoint site.
Check the box saying 'Post to the channel about this tab' if you want everyone to get a notification that a calendar tab is now available.
You will now see a new Calendar Tab in the top navigation bar.
This Calendar will be visible to all the members who are part of this Teams Channel.