A defendant who has pleaded guilty to involvement in an alleged East Coast drug ring told an Alexandria jury yesterday that he frequently bought and used cocaine with five coleagues now on trial in U.S. District Court.

Alfred D.C. McCoy, a music promoter from Winston-Salem, N.C., appearing for the prosecution, was the first witness other than police investigators in the two-day-old trial to testify he had seen the five defendants actually using cocaine.

McCoy, 428 peppered his testimony with an unusual vocabulary of drug-world terms. "I eyeballed the cocaine before I freebased it. . . . I was concerned about getting it up front," McCoy told the jurors, who leaned forward in their chairs to follow his testimoney.

Under prodding from Judge Oren R. Lewis, McCoy explained that "eyeballing" means personal inspection and that "freebasing" is a method of heating cocaine so it can be smoked rather than inhaled. McCoy added that freebasing is a "novel" method of using the illegal drug.

McCoy and 10 others were indicted last May in connection with the case, which prosecutors have described as one of the biggest in the Washington metropolitan area. McCoy, who still faces sentencing, said yesterday he pleaded guilty to one count of using a telephone to distribute cocaine because "I didn't want to go tojail."

Since their indictment, McCoy and two others have pleaded guilty, charges against two people have been dropped and one person, known only as "Carlos," is still at large. All five remaining defendants are charged with conspiracy and distribution of cocaine from July 1978 to May 1979.

McCoy testified yesterday that he had given defendant Michael F. Tillery cocaine to test for purity, had purchased one ounce of cocaine for $1,700 from a second defendant, William Barry Robinson, and had held numerous conversations with defendants Paulette Ashton and Wayne McNair Hargrove about cocaine purchases. The fifth defendant is Marian Teresa Addair Starr.

The prosecution rested its case yesterday after presenting 10 witnesses in two days of testimony.

Conspiracy and distribution charges each carry a maximum penalty of five years in prison and a $15,000 fine. The defense is expected to begin today.