The only legal way to pool hop is if your staying at a sister Resort. For example if your are staying at POR and your friends are staying at POFQ, then you can pool hop between those Resorts only. It's also like that for the Yacht & Beach Club and the 3 All Star Resorts. However if you were staying at Coronado Springs and they were staying at Caribbean Beach then neither one of you can pool hop.
Besides that the only guests who can legally pool hop are DVC members staying on points, however they can not pool hop at SAB at the Yacht & Beach Club or the pool at the Animal Kingdom Lodge. However they are allowed to try the other Resort pools under the circumstances I mentioned. Plus the only time they can not pool hop at all is during the holiday seasons at WDW because those dates are blacked out.