In this example, who are they?

Gordie and Horace were jamming, trying to work out the chorus to a new song. Steve, Buzz, and Eliot joined in, and they finally got the sound right.

Maybe the writer meant this:

. . . Steve, Buzz, and Eliot joined in, and Gordie and Horace finally got the sound right.

Or maybe he meant this:

. . . Steve, Buzz, and Eliot joined in, and the three of them finally got the sound right.

Or possibly this:

. . . Steve, Buzz, and Eliot joined in, and the five of them finally got the sound right.

Any of the three revised sentences is fine, as long as it says what the writer meant.