This was a horribly thought out plot point. Although the novel gives some explanation, the movie never makes it clear whether Sybok is simply convincing people through smooth talking, controlling them through some type of telepathy, or a combination of the two.
Regardless though, we see that out of a crew of 430+, Kirk and Spock are the only ones able to resist Sybok's control outright, with McCoy coming around once he sees Kirk and Spock resist. We see Scotty acting normally, but only because he wasn't around Sybok at any point to be controlled. Everyone else was apparently co-opted without difficulty, including the rest of the command crew, which strains credibility beyond belief.
We've seen what Vulcan mind powers can do in Trek a LOT of times (most similarly in Voyager's "Investigations") so I have no problem whatsoever with the crew falling to Sybok's will. Kirk and Spock being the only ones able to resist.... it's Star Trek! Of course it's gonna be them who can resist, and not Ensign Bloggs from deck 36