Yes, COSIGNERS is a Scrabble word!

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

Is it a valid word?

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

one of two or more signers of the same document (as a treaty or declaration)
a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)