This feature request is working its way through the backlog queue. No new details are available yet.
An error occurred while saving the commentAJ 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.
The team continues to make progress on this. We will share more details as we have them.