How to copyright song with co-writers?

[hitfactory]

I wrote and own the copyright to an instrumental song which I posted to youtube a long time ago. A female singer/songwriter emailed me through youtube about putting vocals to the song. I agreed, through email, to let her add vocals to my music without significantly altering my music. I never agreed to allow her to use the song for commercial purposes, but Im worried that because I agreed to let her add vocals to my song that she can now claim copyright to the "new" song and sell it. Can she do this without any legal stipulations regarding an agreement for dispursment of funds from the sale of the song between her and I?

[FlyingRon]

She can't do it period without your permission. If she's put it up on the web somewhere like youtube, file a complaint with them on the copyright infringment. The fact that she added words does not diminish your rights to the music.

I'm not even sure compulsory license applies in this situation. Are you represented by one of the PROs as well?