The Prime Focus Spectrograph (PFS), a massively multiplexed fiber-fed spectrograph to be mounted on the 8.2-meter Subaru Telescope, will be able to obtain spectra for more than 2,000 stars at once over a ~1.3 degree field of view. The PFS Galactic Archaeology group is proposing to observe six of the dwarf spheroidal satellite galaxies of the Milky Way. In each of the dwarf galaxy fields, likely member stars must be identified and fibers assigned to them. We have developed a novel approach to determining which stars are likely to be members of a given dwarf galaxy, based on pre-existing photometric data. Here, we present the resulting hierarchical Bayesian mixture model technique for evaluating the probability that a star is either Milky Way foreground or a member of the dwarf galaxy target. Our membership probability software will be open-source and available to the community. This work is supported by the generosity of Eric and Wendy Schmidt, by recommendation of the Schmidt Futures program.