Person A: Lets grab lunch with a few co-workers.
Person B: You treating or is the company paying for it?
Person A: Ehh, No its P.Y.O.B. Pay your own bill.
Person B: I'm not going if its not free!
Used in paintball to describe weather you can bring your own paintballs to the field, or if you have to buy the field's paint.There is usually a fee for this - fields have to make money.