How to Add Folders to the OS X El Capitan Sidebar
Adding whatever folder you like to the Sidebar in OS X El Capitan is easy. All you need to do is select the item you want to add and choose File→Add to Sidebar from the menu bar (or press Command+Control+T). You can now reach the item by clicking it in any Finder window’s Sidebar. And you can move files or folders into that folder by dragging them onto the Sidebar icon for the item.
You can also add folders (but not files) to the Sidebar by dragging them onto the Sidebar.
Be careful not to drag it onto another folder or it will be moved into that folder instead of being added to the Sidebar. You’ll see a little line above or below existing folders in the Sidebar; that shows you where this folder will appear if you release the mouse button. If a folder in the Sidebar is highlighted and you don’t see the little line, releasing the mouse button will not add the folder to the Sidebar, but will move it into the highlighted folder.
To remove an item from the Sidebar, right- or Control-click the item and choose Remove from Sidebar.