I have known a number of intelligent people who are Jehovahs Witnesses. I dont think that just because someone is intelligent they are immune from indoctrination. Indoctrination takes place no doubt for a whole host of reasons personal to the individual and it often has absolutely nothing to do with intelligence.
It also begs the question as to how you define intelligence; it is apparent in all sorts of shapes and sizes with some being more academic and scholarly whilst some have a more practical form of intelligence.
This being said, when someone is more academically intelligent and are able to think critically this does provide a barrier to indoctrination unless of course other psychological factors are manipulated and the individual for one reason or another prefers to delude themselves. Often intelligence of this nature is repressed by the organisation especially when they associate independent thinking with something negative such as rebellion or even stupidity.
There are a number of highly intellectual people on this forum and its a pleasure to be surrounded by them and supported by their knowledge and expertise. They have already been mentioned elsewhere and know who they are.
Gary