This feature request is working its way through the backlog queue. No new details are available yet.
-WarrenAJ Kelly commented
My company uses a SharePoint site collection to organize collaboration, files, calendars, etc. based on corporate division and project team. To see real value in Teams, we need to either be able point at a specific subsite (configurable when making the team and editable later) or to be able to include specific document libraries, calendars, and the like from the "Add a tab" feature.
Thanks to everyone who volunteered for testing the Office 365 group calendar. The TAP team will be reaching out as we are closer to being ready.