Can you hire someone to grill food at a big BBQ?!
Anna, check these people out. I don't think they'll come to your house and cook it...but it's really good BBQ.
Yes, in fact I would love to be doing BBQ/Smoking for a living as I love to grill. You didn't mention where you live, but some catering places will also to an outside BBQ just like a regular catered event.
They usually don't use your grill but bring their own as they have to abide by local food laws and health regulations.
Yes. Depending on what city you are in, you may have craigslist. If you do put an ad up in jobs or gigs for the day, how much you will be paying and what it entails. People will respond and you quick the most qualified. Much cheaper than getting a caterer to come
Go to a temp agency that does hospitality work
Used to do temp cheffing
Here is a thought. Do you have a BBQ place near you. Ask the manager if one of the cooks would like a freelance job and cook for your party. I can't imagine if a cook would not love to do this for cash.