Recently, there was an ongoing discussion about whether the OT saints had the Holy Spirit indwelling and Isaiah 63:11 was used as a proof text to suggest that it was indwelling in Moses, which, I think, is how the KJV translation represents the original language using "him" while others offer "them" which helped to include suggestions that Holy spirit was 'among' them. The unique thing about the original Hebrew is that, if two words are spelled exactly alike, or even very similarly, they are closely related in one way or another, whereas other languages such as English the same word can have more than one totally unrelated meaning.
So, to help clarify my understanding, and dispel the confusion, I thought to look up the Hebrew for "in the midst" and the, if I recall correctly, 29 other occurrences of the same word. What did I find? Sometimes it meant "among" and sometimes it meant "within". Although, it did imply "among" far more times than it did "within", it was still an inconclusive finding as to the as precise as can be meaning.
But, I entertained my propensity to be just obnoxious enough to post the passage in original Hebrew, if only to drive home the question of whether we truly 'know for certain' exactly what it says as some might so often insist. However, what gave me the best insight as to the exact meaning in the context of that verse came two lines before in verse 9 with the reference to "the angel of His presence [who] saved them."
Still, whether within (me) or among his people (you all), those two ideas that He is with us (Emmanuel) brings me an tremendous amount of comfort.