In August of 1993, Kent Beck and Grady Booch sponsored a mountain retreat in Colorado where a group converged on foundations for software patterns. Ward Cunningham, Ralph Johnson, Ken Auer, Hal Hildebrand, Grady Booch, Kent Beck and Jim Coplien struggled with Alexander's ideas and our own experiences to forge a marriage of objects and patterns. The Group agreed that we were ready to build on Erich Gamma's foundation work studying object-oriented patterns, to use patterns in a generative way in the sense that Christopher Alexander uses patterns for urban planning and building architecture. We then used the term ''generative'' to mean ''creational'' to distinguish them from ''Gamma patterns'' that captured observations. The Group was meeting on the side of a hill when all this occurred, hence the name.
Since then, the Hillside Group has been incorporated as an educational non-profit. It has sponsored and helped run various conferences (PlopConference, EuroPlop, ChiliPlop, KoalaPlop, Mensore PLoP, SugarloafPLoP, and UP97) and has been responsible for getting the PatternLanguagesOfProgramDesign series of books put together and published.
More history can be found on the Portland's Pattern Repository