Yes, GREENBRIERS is a Scrabble word!

Letter values in Scrabble US: G 2, R 1, E 1, E 1, N 1, B 3, R 1, I 1, E 1, R 1, S 1.

Is it a valid word?

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

a very prickly woody vine of the eastern United States growing in tangled masses having tough round stems with shiny leathery leaves and small greenish flowers followed by clusters of inedible shiny black berries

Also: horse brier, horse-brier, smilax rotundifolia.