Create a Shared Calendar in a Microsoft Teams Group

Add the SharePoint site to Team Tabs. Click the + icon after the Last tab in the team Space. 

Add SharePoint Site as a Tab in Teams

In next dialogue Select the SharePoint Home

SharePoint Site added as a Tab

Access the SharePoint in the browser. Click the small globe Icon on the top right corner in the team space. This will open the SharePoint site in your default browser

Team’s SharePoint Site in Browser

Create the Calendar App in the SharePoint Site.
Use the + New drop down select the choice App. 

In the Next window, Use the search bar to search Calendar. Click on the Calendar app

Give the Calendar a name in the next dialog box

You have created the Team Calendar. The Calendar will be listed in the SharePoint Site contents list

The New Team Calendar

Copy the URL for the calendar from the browser window - you'll need the team calendar URL in the next step

Adding the calendar as a tab in the MS Team Space. Use the + icon in the team channel to add a new tab.

On the Add a Tab screen, click Website

Give the tab a name and in the URL option give the calendar’s URL. 

Teams Calendar as a tab in MS Teams

OPTIONAL: Connect the calendar to Outlook. In this last step we will see how to connect this SharePoint calendar to your outlook. Open the calendar in browser and click Connect to Outlook icon from the Calendar tab in the site

Connect to Outlook - Click Open Outlook in the alert pop-up dialog

Click Yes in the Security notification pop-up in Outlook. Now the Calendar will be added to you outlook.

Outlook alert to add the calendar

Adapted from https://medium.com/@saranyansenthivel/creating-a-shared-calendar-in-microsoft-teams-400ab556bfa3