What should i charge just for grilling?

$50 to $100 per hour of your time.

I agree, Figure out what your time is worth, that said, charge for the whole day not just the time you are there. I charge per day, but I will stick around and make it worth the customer's time.

I see problems bigger than a payment.

Are you purchasing the meat and getting reimbursed? or are you showing up to cook whatever they happened to buy? If they buy inferior product what will it do to your reputation? Are you going to show up and have them hand you tri tips that are frozen as hard as a bowling ball? I am not trying to be a smart ass----but I have had all of these things happen.

When people in line get a 1/10 pound chicken weiner---they do not care who supplied the meat----they look at the name on the BBQ pit. If the condiment table does not have what they want----they look at the name on the BBQ pit. If you cook boneless skinless thighs to perfection, and hand them to someone to put in a chafing tray to try to keep for several hours---the people look at the name on the BBQ pit.

Protect your reputation---ask questions. What meat is being supplied---ask specifics. Are you supplying spices and sauces? Are you trimming the tri tips?

Figure out what your time is worth. You have prep time---moving BBQ pit time--cleaning out BBQ pit time etc. Do not cheat yourself.

Are there any sides? Who is doing the sides?---if there is a problem with the sides they will be reading the name on the side of your BBQ pit.

A company function?
Just take your daily wage & volunteer to do the cooking.
Less problems that way.

Good---that way you have control over the meat quality, and if the people supplying the sides are also the people that are eating them, then they will know where they came from.

Just remember that catering and vending food is a form of gambling that is allowed in all 50 states---keep the odds in your favor.