Its free, with you staying at a WDW resort all you have to do is show them your room key. When the time comes towards the end of the night you just show them your key and you get a wristband(theres different areas in the park you have to go for them). That wristband gets you on the rides, if you don't have one then you can't ride them.
And thats the nice thing about staying on property you get to stay in the park longer.