Far too early for that.
Even going by the best estimate of 2017-18 for the IOC of the first squadron (a whole regiment by then seems a little too optimistic in my view), that is still 5-6 years away. A lot of those people might have been transfered/promoted to different positions/units by then, and some might have left the PLAAF
Also bare in mind that this is the first prototype. The pre-production and production models may have a lot of differences, especially if they use WS15s instead of whatever is powering the prototype. Thus starting training of pilots and ground crews now seem very pre-mature. These skills are highly specific and perishable.
Also, these guys are just there to watch, the test flights would be happening whether they are there or not, and bussing them over would not cost that much money or take that much effort to organize, so I don't really see why it would be such a big deal if they just brought a class of freshly qualified pilots from a training facility somewhere nearby to see the J20 take off and do a test flight before they are shipped off to their new regiments.
I fully agree with you that they are not there to start training themselves for the plane, it's far too early for that, and that wasn't my meaning at all. But they are certainly air force, and I can't imagine a bunch of newly qualified pilots (or any other random group) getting to be that close to the prototype just for the heck of it.
So I think they're likely personnel from one of the first (or test) regiment's who are planned to get J-20 just observing a test flight. Maybe they were making a trip to chengdu and were offered a look, or maybe PLAAF sent them there exclusively to watch (for whatever motivation the high brass has -- morale, getting close first hand look, whatever). Imho out of any group in the air force who would observe a test flight like so, I would guess they would be some of the people expected to eventually operate them. That doesn't mean they've already started training, but it is something for the personnel to keep in the back of their mind, like "this is what we're working to in a couple of years".
Although you make a good point about transferring personnel between units and positions. But we don't know how PLAAF works either wrt switching units or allowing observers for test flights of new aircraft. The air force peeps could be anyone.