I don't think anyone much cares if you hang out at the Poly beach for a half-hour to watch the fireworks, but I'm not sure they'd be OK with camping out there for the afternoon if you're not staying at the resort. There are a limited number of lounge chairs, and the paying customers really should get first crack at them. Personally, I wouldn't do it, and I'd be fairly ticked off if I paid hundreds of dollars to stay there and then couldn't enjoy the beach because non-resort-guests were taking up the chairs.
I'm honestly not sure what the official policy is on this, or if there even is one. One afternoon at the WL when most of the lounge chairs on the beach were taken, security did walk down and ask to see resort IDs, and they did ask folks to leave if they couldn't produce one.
My suspicion is that if it's not crowded, and you and your friend are just about the only ones at the beach, no one will say anything. If it is crowded, though, they may well ask you to leave.