I work on multiple projects in Sublime Text and would love the option to select multiple files in the sidebar, right click, and select Group to combine them all together in an easily collapsible group. This would make it easier to switch between my Rainmeter skins, and my HTML files on the fly and keep everything sorted and organized.

You can achieve similar results with project switcher (Ctrl + Alt + P on Windows).

This would indeed be useful. See my homemade solution here.