I'd like to start from "I don't think," but I don't know how.
I think some of them don't know the answer.
How can I convey the same meaning?
I don't think all of them know the answer.
I don't think some of them know the answer.
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Sign up to join this community(1) is the natural way to say it. There's nothing grammatically wrong with (2), but it doesn't sound idiomatic to me. If you wanted to refer to 'some of them', you would use your original sentence.
All of them means everyone of a group of people, some of them is particular ones of them but not all the people