I've just create a new Chrome extension which allows for better control of grouped tabs. Specifically, you can toggle them by pressing Alt+1, Alt+2, etc. The extension also remembers the last active tab in each group.
When you open a group, it will switch to that previously selected tab, or, if none was activated before, it will default to the first one in the group.
When you close all groups, it will activate the previously active tab from outside the groups.
The extension is very early stage and primitive. I want to add a control panel to customise the shortcuts and some of the behaviour, but for now I'm waiting to see if enough people are interested in this idea.
You can find it here: https://github.com/degecko/better-groups
It will soon be published on the official Chrome Web Store.