Did this again since my last post. Once you have scanned all the item displays in the center of Illium (Luminous Gardens), this challenge pops up. Run to the West side of the garden, there are two citizens (human and chua) to your left. Press [T] to cheer to them. There is a 10-second "cooldown" to the citizens (NOT the ability) - that is, wait at least ten seconds and use the ability ON THE SAME TWO CITIZENS again, and you won't have to run around. Seems to be a "hard" 10 seconds, so the best bet is to press [T], look up at the timer on the challenge, and count down to ten. You've got close to two minutes (i think) to do this one. If you mess up, you can always click "Retry".

There seem to be VERY FEW "citizens" in Illium. The only way i found to complete this was to find two groups of two citizens (west side of garden and by the crafting area) and run back and forth between them. By the time you run back to the first two, they can be "cheered at" again. It may be possible to simply wait at one group, but I don't know the exact cooldown - you definitely can't spam the cheer, they simply ignore you.

This is a Scientist only challenge, so don't go hunting for it if you're not one :P