CMR collaborators. Some questions on working with others.

Hey guys and gals, I know that there are some of you out there that work with others on your songs. How do you prefer to work? Do you write full songs and get musicians that play instruments that you don't to help you? Do you have segments of songs and work with others to help complete the idea? Has any one here ever put out a song idea and had others ad parts or write lyrics?
Let me know your thoughts on the topic.