Yes, MATES is a Scrabble word!

Letter values in Scrabble US: M 3, A 1, T 1, E 1, S 1.

Is it a valid word?

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

a pair of people who live together
the officer below the master on a commercial ship
a fellow member of a team
the partner of an animal (especially a sexual partner)
a person's partner in marriage
an exact duplicate
one of a pair
South American holly; leaves used in making a drink like tea
informal term for a friend of the same sex
South American tea-like drink made from leaves of a South American holly called mate
a chess move constituting an inescapable and indefensible attack on the opponent's king

engage in sexual intercourse
bring two objects, ideas, or people together
place an opponent's king under an attack from which it cannot escape and thus ending the game
twist together or entwine into a confusing mass
change texture so as to become matted and felt-like

Also: first mate, married person, better half, paraguay tea, ilex paraguariensis, felt up, mat up, matt-up, matte up.