I'd probably feel a little weird to asking at first too just because I'd be afraid of them saying no, but if that's what you are interested in photographing, I do it. The funny part about nerves is that we make a bigger deal leading up to the said event that once it happened, it's easily forgotten, positive or negative. Most people in general too, are very accepting of letting people do harmless things as take their photo as long as it has some sort of purpose behind it. Especially like a parkour group because they'd probably like the exposure to their group and the sport in general. I say go for it Tim, you have nothing to loss.