Amnith's Invite Broadcaster (amnib by short) is a small addon that does three things:

Broadcasts to the guild chat that invites for the given instance have started.
Invites people whispering invite to you to your group/raid.
Mass-invites people to your group.

Broadcasting can be done by typing /ib start <instance>, for instance:
/ib start Karazhan sends "Invites for Karazhan have started. For an invite, please whisper me 'invite' without the quotes." to the guild chat. To reannounce, use /ib announce.

Invites happen when people whisper you the exact word invite when the inviter has been activated. The inviter saves its state over sessions. To stop taking invites, type /ib stop.

Full command list
/ib start <instance>: Starts the auto-invites and broadcasts to the guild if instance specified.
/ib stop: Stops the auto-invites.
/ib status: Shows the current status of the invite broadcaster.
/ib announce: Announces the previously started instance invite.
/ib massinv <player1> [player2] [player3] [...]: Performs a mass invite for all players passed.
/ib text: Shows current text to broadcast.
/ib text <text>: Allows to set the text for invites. <INSTANCE> specifies the instance, <KEYWORD> specifies the default invite keyword.
/ib resettext: Resets invite text to default.
/ib keyword [<add|del|default> keyword]: With no parameters, lists current auto-invite keywords. Otherwise adds or removes given keyword, or sets it to the standard for broadcasting.
/ib version: Shows you what version you are using.