Yes, PARTIES is a Scrabble word!

Letter values in Scrabble US: P 3, A 1, R 1, T 1, I 1, E 1, S 1.

Is it a valid word?

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

an organization to gain political power
a group of people gathered together for pleasure
a band of people associated temporarily in some activity
an occasion on which people can assemble for social interaction and entertainment
a person involved in legal proceedings

Also: political party.