Both are grammatically correct (depending on the rest of the sentences they're embedded in), but only #1 makes sense in this case.
Let me give a different example that might help more:
A. The employees have computers.
B. The employees have a computer.
In example A, the employees have multiple computers. This often, but not always, implies that the employees each have at least one computer.
In example B, the employees jointly have at least one (but probably just one) computer. But this does not carry the each implication that example A does.
When it comes to IQ, multiple people cannot jointly have IQ scores, so the type of construction in example B doesn't make sense. You use the type of construction in example A instead, and the reader/listener will naturally understand this as meaning "people who each have a high IQ".
See below for Edwin's notes in the comments about idiomatic usage, I may be coming at this from a bit of a purist angle.