Almost there Rated 4 out of 5 stars
Features to consider:
(1) The ability to CTRL + click tabs (selecting them) before choosing to group them.
(2) The ability to save tab groups and recall them at any time (maybe via the tabs, right click menu).
(3) Groups can be expanded either horizontally, vertically (tab explorer bar), or in a drop down list which disappears when mouse is moved away*** (my preference).