take a look at paypal masspay - there is a surcharge for using it (1%
of the transaction, capped at 1USD if I remember correctly). You'll
need to take the surcharge into account when figuring out how much to
pay people (it's added to your debit rather than subtracted from the
amount they are creditied).
I'm not aware of any rails code to do it but it's a simple upload of a
(iirc) tab delimited file describing what payments you want made.