The most important thing for me is to have somewhere quiet I can go. I already feel shitty having a panic attack, seeing the pitying looks only makes it worse.
Then, I will usually try to manage it on my own. First by breathing, then by meds. Only if both of those don't work I will seek for outside help, so I recommend you asking the sufferer if they actually want help. Maybe they would prefer to be left alone, and you should respect that.
Breathing exercises, grounding is good. What works for me the best however is either talking about it, or, if I don't feel like it, getting distracted. If the second, I don't care what I'm being distracted with, so long as the person talks. I've looked at prom dresses, heard a colleague talk about his music ambitions, etc.
All in all, asking HOW you can help will work. People have different coping mechanisms, and just having someone who cares enough to see you through a panic attack is already hugely relieving.
Finally, please don't touch people if they don't ask for it. If they're crying, offer a glass of water or hand them a tissue. Panic attacks have the sufferer feeling very vulnerable and touch can actually be harmful. (Plus, you're at work.)