I was there. The storms were intense, but fortunately brief.
They had everyone take shelter, either in a permanent building or a closed vehicle, as there was a lot of lightning in the area.
The tech/media building, shower facility, medical building and even the museum were opened up for fans to take shelter.
We were seated overlooking turn 2, and had onsite parking, since my father-in-law volunteered. The car was pretty close, so that's where we went to wait it out.
At some point, the rain was coming sideways due to high winds.
As soon as the lightning was out of the area, the circuit sent out equipment to start removing standing water at a few key spots, namely Turns 1, 9 and 12.
Here are my Google Photos galleries of the IndyCar/MRTI/PWC weekend at Barber, for anyone who is interested.
Friday:
http://bit.ly/1ikLuNG
Saturday:
http://bit.ly/1fHU6Jr
Sunday:
http://bit.ly/1fHXqEv