If there is a group of people on the ramp watching airplanes, the FAA has considered that group to make the area “congested.” That triggers the clearance distances of 14 CFR 91.119(b).
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91.119 Minimum safe altitudes; general(b) Over congested areas – Over any congested area of a city, town, or settlement, or over any open-air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft.
When the altitude of an airplane is less than 12,500 feet, there is no supplemental oxygen required for anyone in a private plane. From 12,500 feet to 14,000 feet, supplemental oxygen must be used by the required flight crew for any portion of the flight that is more than 30 minutes.