Yes, TRUSTEE is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, U 1, S 1, T 1, E 1, E 1.

Is it a valid word?

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

a person (or institution) to whom legal title to property is entrusted to use for another's benefit
members of a governing board

Also: legal guardian.