Yes, SUBSCRIBE is a Scrabble word!

Letter values in Scrabble US: S 1, U 1, B 3, S 1, C 3, R 1, I 1, B 3, E 1.

Is it a valid word?

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

offer to buy, as of stocks and shares
mark with one's signature; write one's name (on)
adopt as a belief
pay (an amount of money) as a contribution to a charity or service, especially at regular intervals
receive or obtain regularly

Also: subscribe to.