I had to do this once. Basically you're there just to enforce safety rules. Goggle rules should be the same. We had them use standard full face, full ear protection paintball goggles, as per our insurance, though some places allow just goggles (I wouldn't, but that's me). We also had them use barrel covers off the field. Don't worry about calling out hits of course, they're strictly on an honor system. There is a potential for arguments ("I hit you", "did not", "did too"), so pay attention, but I didn't have any issues.
No industry standard for FPS yet, but I've seen 350 fps and 450 fps mentioned a lot, with the usual 10 foot minimum distance. I'd go by your field layout, if there's a lot of close shots (indoor field, small speedball'ish field, some urban fields, etc) I'd go lower.
Some places have a "sniper" velocity for single shot bolt action guns of 550-600fps, but they have a minimum distance of 100 ft. If you allow this you're going to have to keep on your toes to enforce it.
You may want to do a little experimenting before hand if the field they're going to use has netting. Some netting stops airsoft pellets, some don't. We had to rope off a section of our staging area due to netting that was too open to stop the pellets.
Here's a different local field's airsoft rules, might be some useful stuff for ya...