Tell them next time that the Owner said we can be there and unless he complains it is perfectly within our rights to hang out in a place where the owner says we can. Thats like if I had a huge driveway and we all parked there and someone complained: unless I was the one complaining, they cannot legally make you leave unless you are in violation of the noise ordinance, which we never once were considering most of us left shortly after 10 usually.