I'm not really sure, but after looking at some example sentences in Jisho.org, I came to the conclusion that in order to say, for example, "Out of all of the people that I know", you would say 「全部の知っている人たちの中で。。。」But is 「全部の知っている人たちのうちで」also correct? Also, why do we use 「で」instead of 「に」in these sentences and do we even have to put a particle after うち?
First, 全部の人 is an obscure expression because 全部 is a counter for objects* (that said, 私が知っている人全部のうち sounds to some extent better for some reason), so I'd translate "all of the people" to 私が知っているすべての人 or 私が知っている人全員、私が知っている人すべて.
As for your question, yes, the sentence with うち is correct too and you can omit the particle で after うち.
If you use に instead of で, that is, 私が知っているすべての人の 中/うち に would be "into all of the people that I know".
As for the question what if you use に instead of で, well, it depends on what predicate comes after.
- edit: not so much 'objects' as 'portion'.