Guild Whisper Invite allows you to define a keyphrase, that when whispered to you, will automatically invite the person who whispered the phrase to you to the guild, assuming you have guild invite privileges.
It was written on request for Thunder, on the wowace suggestion forums.
Use /gwi or /GuildWhisperInvite to access the text-based menu.
/gwi phrase <thephrase> changes the keyphrase to the phrase of your choice