i dont always agree with james, but he's correct on this one
When you look at the actual text, it turns out to be a bit of a mistranslation anyway.
The Interpreters Dictionary of the Bible: Although frequently cited as an example of Jesus mysticism or inwardness, this interpretation rests chiefly upon the old translation, within you, [KJ, Dy] understood in the unfortunate modern sense of you as singular; the you ([hy‧mon′]) is plural (Jesus is addressing the Phariseesvs. 20) . . . The theory that the kingdom of God is an inner state of mind, or of personal salvation, runs counter to the context of this verse, and also to the whole NT presentation of the idea.
The idea comes from how the old english tranlsators rendered the verse as 'within you'... Jesus was addressing the people as a whole. The way we understand this verse is that Jesus, who was himself the King of the kingdom, was among them....he was there in their midst and its in this way the 'kingdom was among them'
Jesus was the realisation of the heavenly kingdom so quite literally, the kingdom was among those people.