How to Share?

I have a situation that I am wondering how others may have solved. I have an open source (BSD) project whose copyright is owned by the company I work for. Another person, working for another company (in another country!) wishes to contribute to this project, which I think is great.

How is the ownership of copyright usually sorted out in these situations? Is it shared by both parties? Does each party maintain copyright to their respective contributions? Does the party wishing to contribute sign over copyright ownership? Is there another alternative that I haven’t thought of? Surely this situation comes up a lot; how is it usually handled?