Comment 7 for bug 729357

Check out the /usr/share/indicators/messages/applications/ folder. It looks like in the case of Pidgin, a simple pointer is created here and a shortcut is then automatically added to the bottom of the messaging menu without anything else needed to be added to the .desktop file.

It looks like the evolution.desktop I posted in the previous message shows you how to add groups to Ubuntu's messaging indicator which may not be what you want.