A "Ground Stop" is an ATC tool to keep air traffic at manageable levels. When there is a ground stop in place for an airport, aircraft destined for that airport are not supposed to depart until the ground stop is lifted/expires. This doesn't preclude ATC from making exceptions as needed. It is difficult to say what happened in your particular circumstance, but there are several possibilities that are more likely than others such as dispatch calling ATC and begging to get a particular flight off the ground for some logistical reason, someone on your flight knew someone in the ATC system and was able to negotiate a release, or someone in the tower at your departure airport screwed up and released your flight while the GS was in place.