If you listen to the tapes (which, totally understandable to not) - there was a woman (iirc) who argued pretty vociferously against Jim Jones plan. They had killed a U.S. congressman at that point - the idea was that they were doomed anyway. But people did argue against him.
Another aspect is that they had started the suicide drills long before they had even moved to Guyana. They had church services in California where they were told to drink what they were taught was poison.
People who didn’t drink the Flavorade were shot. You don’t need everyone to go along with it - you just need a handful of the people who are willing to take quick, immediate action.
The recent Behind the Bastards episode about Kevin Smith (not the director of Dogma but a Jamaican Pentecostal preacher) makes this pretty clear - there’s these fucked up dynamics which make certain people more willing to act, and others less so.