InviteMe allows other people to invite themselves to your party.
After installation 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. People cannot use it to invite themselves to raids.
There is another addon of the same name (that is currently most likely broken) with similar features located here, created by Striph.
-- 70000.1: ToC update. Check for name returning nil.
-- 60200.1: ToC update.
-- 60000.2: Now works with Real ID friends!
-- 60000.1: Back from the dead! ToC update. IsPartyLeader -> UnitIsGroupLeader. Ambiguate names.
-- 40000.1: ToC update. Reworked code and removed in game configuration.
-- 30100.1: ToC update. Added license.
-- 30000.1: Works fine in Wrath.
-- 20300.1: ToC update.
-- 20100.1: ToC update.
-- 20003.1: It starts. (Unreleased)