TeddyParker and CycleChic - note that this is NOT new behavior at HP. This has been going on in this park for at least 10 years (or as long as I have been riding there). There is similar activity at the Middlesex Fells reservation.
The rangers in both cases are VERY aware of the issue, and are working hard to try to curtail it. Keep riding in the park, and if you encounter something like this again, report it just as you have this time.
I usually ride out of the Jenkins lot as well, and haven't seen this so much up at this end of the park. When I first started riding there, it was REALLY bad in the first big lot down by the police barracks - trollers everywhere, and you could tell who they were. We stopped parking there and went up to Jenkins (all the good singletrack is up that way anyway

).
I have a lot of friends who ride there on a regular basis, and have not had this kind of encounter, so I wouldn't think you would expect to see something like this on a regular basis. The other thing is, that as a female, you're not their type, so you shouldn't have to worry for your safety

. Also, as someone else pointed out, there's a good chance that these guys are going home to families, so they want to be as anonymous as possible.
I hope this hasn't scarred you too badly, and that you'll keep going back! HP is an awesome place to ride.
SheFly