First -- to the OP: I completely understand where you're coming from -- but I tend to agree with the folks here who say it was not your fault -- Family hour or not, blind curve or not, you have no reasonable expectation that a group will block the entire path (and yes, I've seen it on the MUP here, and yes, it irritates the beejezus out of me!) And they had EVERY obligation to move, or at least make some effort.

Second -- to TJSPIEL: When I first started commuting, I wondered at the guys (usually roadies) who bailed off the MUP onto the street at one point on my usual route... right up until the first time I had a car back out of a driveway into my path (this particular stretch is parallel to a residential street, with road on one side, and homes on the other -- i.e. glorified sidewalk -- and is a pretty decent downhill). I still ride on the MUP, but I do it with hands on the brakes, eyes on the driveways and at much lower speed.