"ke beech" - among; "ke beech meN" - in the middle of, among.
Mostly interchangeable. For sentences where "in the middle of" can only work and not "among", one would use "ke beech meN": for example, "chaadar ke beech meN ek phool kaaRh do". If you mean the very center, you can say "beech-o-beech (meN)".