I would agree with the LL if all of you signed one agreement or if each agreement specified that each tenant was joint and several (google it) with every other tenant but you haven't indicated that the second part of that sentence applies.
Seems to me that the LL is renting to different people with individual contracts and owes you your security deposit.
Study the security deposit sections of the landlord tenant statute:
http://apps.leg.wa.gov/RCW/default.aspx?cite=59.18
Then I suggest you send him a demand letter with a deadline date to comply, quoting the appropriate parts of the statute, and sue him in small claims court if he fails to refund the deposit.

