Whenever I start a game and invite friends, if I drag their name to the invite list, it doesn't show up. It removes their name from the Available Players list but doesn't add them to the invite list.
Using the + sign next to their name does add them to the invite list though. Just thought I'd mention it, if it is a bug or not.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.