The Harpers are... well, they seek a world where people need not fear, where song and history are kept and not lost or twisted to promote a particular belief, where nature and civilization are in balance. What that tends to mean in practice is a lot of running around and fighting those who believe otherwise, like slavers, and crime syndicates, and the less pleasant churches - Bane, god of Tyranny, as an example.

You don't really choose to join; you get recruited after coming to the attentions of other Harpers. There's no formal swearing of oaths until you get your pin, showing you've got enough sense to not leave a mess behind you.

[She unbuttons her vest (there is a shirt underneath) and flips it over, showing a pin in the shape of a crescent moon with a harp between the horns, made of a silver metal, then flips it back.]

In general, the Harpers let people do what they feel they should do, but sometimes you get an assignment. Last one I got was... I think I mentioned it, starting a slave rebellion.