Yes, INTENSIFY is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, T 1, E 1, N 1, S 1, I 1, F 4, Y 4.

Is it a valid word?

INTENSIFY is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

increase in extent or intensity
make more intense, stronger, or more marked
become more intense
make the chemically affected part of (a negative) denser or more opaque in order produce a stronger contrast between light and dark

Also: step up.