That person is on PC!!

[Hybrid Trainer]

I'd find out their username, create and alt account and then send them a PM describing what they look like at that moment in time and advise them to start running.

Unless they were cute or someone I recognised, then I'd just go over and say "Excuse me, but you're x on PokeCommunity right? I'm Hybrid Trainer, nice to meet you."

 

[Synerjee]

I'd be really excited! But I won't directly approach them (seeing that I'm quite a shy person in real life). I would try to glance over at the person's screen to catch that person's username. If I manage to see it, I'll go to that person's profile and leave a VM, saying "Hey, I don't know you, but I was just walking past you and happened to see your username. I think we're sitting in the same room. You're at (gives location of where the person's sitting) right? :P", and then see how things go from there.

 

[Twilight Sky]

Might as well say hi! It might be a once in a lifetime opportunity, so I might as well the chance to get to know who they are! :3 We might become PC buddies, who knows~!