Ensemble is a World of Warcraft addon that will automatically accept group requests from people on your friends list, RealID friends, and fellow guild members.
There is nothing to configure -- the addon will either auto-accept a group invitation or not. If the group leader is not a friend or in your guild, Ensemble will automatically perform a /who on the person.
I think I originally got the idea for this addon several years ago from Tekkub over on the WoWAce forums (he definitely had the idea to do the /who for an unknown person). I was self-studying iterators at the time and wanted to create a practical use for them with the generic-for statement in Lua.
The script currently is commented so if you're interested in how to write an iterator, give the above link and my script a read.