Yes, ALTOGETHER is a Scrabble word!

Letter values in Scrabble US: A 1, L 1, T 1, O 1, G 2, E 1, T 1, H 4, E 1, R 1.

Is it a valid word?

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

to a complete degree or to the full or entire extent (`whole' is often used informally for `wholly')
with everything included or counted
with everything considered (and neglecting details)

Also: birthday suit, all told, in all, all in all, on the whole, tout ensemble.