The League of Lost Penstari is a Molnari overarching confederation of smaller fraternities, all associated with a fallen Barathi Noble House. Any individual that can claim an association with a fallen House (however tenuous, like "My grandmother was a scullery girl in House Mellis") can join its sodality (for example, "the Sodality of Mellis"). Sodalities do charitable works for their members and compete against other soldalities in various ways (sports and other competitions, mostly).
That's what they say, at least.
Some people whisper, however, that there is more going on under the surface. Hidden Scions of Noble Houses thought "regrettably drowned," still alive… still plotting a reversal of their fortunes, regaining their Penstari, and returning in triumph to resume their place on Barathi.
Who knows if the rumors are true?