Well, the pool area could be an issue. The pools at the resorts are for the guests at that resort..not the guests of the guests. But, I'm sure that if you check at the front desk, they will give you permission.
As far as parking goes, you could get a parking permit for them when you check in. They can give your name when they get to the guard shack, then you can give them the permit to park so they can use it to hang out with you.
Before anyone starts on me about them not be registered guests (I am a strong proponent of not allowing non-guests use the parking lots for the resorts), I figure that they have family staying there...it would be pretty much the same as if the mom/dad had a car.
Yes, they can walk around the boardwalk with you, as well as walk to the parks. Anyone can do those things.