InviteMe allows other people to invite themselves to your party.
-- Enabled by default. Anyone can now whisper "!inviteme" to you to invite themselves to your party.
-- There is a simple configuration section at the top of InviteMe.lua.
There is another addon of the same name with similar/more features located here, created by Striph. I make no claim as to which is the better addon, just a heads up to avoid confusion.