Bill Cosby -- New Allegation ... He Slipped Me A Quaalude and I Woke Up with His Friend

Bill Cosby slipped a close friend a Quaalude and hours later she was in bed with one of his buddies ... this according to the woman.

Joyce Emmons -- who ran comedy clubs in the '70s and '80s -- tells TMZ she was close to Cosby in the late '70s and often hung with him in Vegas where he performed.

She says although she never saw Bill drink or do drugs, he often had "a drawer full of drugs," including Quaaludes, in his hotel room. She says women were frequently around Bill and would often take the drug knowingly and voluntarily. She also says men partied with the drugs as well.

Emmons tells us ... one night she got a bad migraine and Cosby offered her a white pill which he said "was a little strong" but could cure a headache.

She says she took the pill, blacked out, and the next thing she knew she was nude in bed in Cosby's suite with one of his friends -- a guy who had unsuccessfully tried hitting on her earlier in the evening.

Emmons says she confronted Bill and demanded to know what drug she took, and he laughed and said it was "just a Quaalude."