I believe this is R.A. Lafferty's short story "Sky" (1971), first published in New Dimensions 1 and then collected in Nebula Award Stories 7, which had a SFBC release (as Nebula Award Stories Seven) in 1973.
The drug is called "Sky" and the people who take it and jump are called "Sky-Divers."
"A sack of Sky from the nervous mouse. Jump, or the sun will gobble your house!" Welkin sang-song, and she was already higher than most skies.
"Hurry, hurry!" the Sky-Seller begged, thrusting the sack to her while his black eyes trembled and glittered (if real light should ever reflect into them he'd go blind).
Welkin took the sack of Sky, and scrambled money notes into his hands, which had furred palms. (Really? Yes, really.)
A small group of them go Sky-Diving:
Four of them when Sky-Diving that morning. Welkin herself, Karl Vlieger, Icarus Riley, Joseph Alzarsi; and the pilot was - (no, not who you think, he had already threatened to turn them all in; they'd use that pilot no more) - the pilot was Ronald Kolibri in his little crop-dusting plane.
The story is a bit confusing, since it's sometimes hard to tell how much we're getting simply from their drugged perceptions, but it seems that time stops:
"We have our own rotundity and sphere here," said Icarus Riley (these are their Sky-Diver names, not their legal names), "and it is apart from all worlds and bodies. The worlds and the bodies do not exist for as long a time as we say that they do not exist. The axis of our present space is its own concord. Therefore, it being in perfect concord, Time stops."
All their watches had stopped, at least.
"But there is a world below," said Karl. "It is an abject world, and we can keep it abject forever if we wish. But it has at least a shadowy existence, and later we will let it fill out again in our compassion for lowly things. It is flat, though, and we must insist that it remain flat."
"This is important," Joseph said with a deep importance of one on Sky. "So long as our own space is bowed and globed, the world will remain flat or depressed. But the world must not be allowed to bow its back again. We are in danger if it ever does. So long as it is truly flat and abject it cannot crash ourselves into it."
"How long could we fall," Welkin asked, "if we had not stopped time, if we had let it flow at its own pace, or at ours? How long could we fall?"
