I've done this. Since the they are paying for the ride there are some
things you must do. You need to do two things...
1) If you are a private pilot you need notify the FSDO of your
intentions and send them the information the FARs require (log book
showing minimum time, currency, etc) See 61.113(d) for required items
to supply the FSDO.
2) Go to the AOPA website and submit an application for a drug test
exemption.
3) You will need to sure that you do not have more than 100 hours since
annual on the plane (otherwise you'll need a 100hr inspection).

You will need to have the non-profit 503c number of the charity as
well. All this is much easier if you are commercial because you can
skip step1.

Cheers
-Robert