Ya well I am only 14 and just starting out, it seems like that is a big group of really good guys.
And they are a group of really good guys. If it were me, and I had the opportunity to practice/train with a group of really good/experienced guys, I'd take it.

Otherwise, it sounds to me like what you're saying is, "I don't feel comfortable training in public, but it's not a good idea to train at night, and I don't feel comfortable training with an established group, but there's no one else around to train with." If all of this is true, maybe you should just give up and quit parkour now. I mean, it's really too hard, right?

Okay okay, I'm kidding.

But the point is, it's unlikely that you'll find the ideal training situation ever. So do the best you can with what you have to work with. I used to feel silly training in public too, when I first started out. In a lot of ways I still do! I worked hard to find a group and that is moving along slowly, but surely. But to me parkour is important enough that it's worth the difficulties that come with training alone, or trying to get a group organized. After all, feeling silly training in public is just another obstacle, right?
I suspect, although I'm not familiar at all with the scene in SF, that if you joined and asked those guys for help with training, they'd be pretty friendly and supportive. You don't lose anything by trying, do you?
As a side note, I don't think you should be monkeying around on rooftops at all, but that's just me. I'm kind of a "den mother" type.
In any case, I hope you come up with a workable solution for your dilemma soon!
Good luck!