An unknown hero, an ad genius, is telling his story about working in Coca-Cola:
"Somehow, some way, my songs reached Mr. Goizueta’s desk and he personally called and asked me to come to work full-time for Coca-Cola in Atlanta in their marketing department. Most of the songs were mine after that, worked on with a big ad agency named McCann-Ericsson and a fellow called Billy Davis."
Does "worked on with" mean that his songs were developed by McCann-Ericsson and Billy Davis or that they were written together?
That is to say "his songs were developed by McCann-Ericsson and Billy Davis". Thank you.