After installing Groupy 2.3, I tried it out on eM Client, opening the main mail window and the calendar window. Great, it puts them into one window with two tabs. Okay, go and do some tasks in a browser, then press Alt-Tab to return to eM Client. Ah - it's not showing in the Alt-Tab list. But it is in the taskbar. With Word, on the other hand, if I open several files, it displays the different tabs separately in the Alt-Tab list.
The behaviour above is if I open the Calendar window from within an existing eM Client window. If, on the other hand, I right-click on the taskbar icon and choose "Calendar", the two windows still get combined into a tabbed window, but they also appear separately in the Alt-Tab list.