Shortcuts to Group calendars in drop down menu

Does anyone know where this feature is and how to customize it? One of our users has managed to put some of the group calendars she looks at into the drop down menu from the calendar. I have also managed to do this but dont know how. I created a test group calendar and then opened a few more. For some reason the test one shows up in the drop down but none of the others do. I have looked at the prefrences for opening group calendars but it says nothing about keeping shortcuts listed in the drop down. Nor does the help file.

This is quite a handy feature but im unable to find any info on it.

Anyone know ?

Thanks in advance