I'm just wondering what peoples opinions are in regards to sharing credit on songs.

Just a few instances..
If you write a song and the music is all yours, you've written the chorus for the song but somebody else comes up with a first verse that takes you in a different direction. They also sing it. How would you credit that person?
Would they be entitled to a share of the royalties if the song made any money?

What about if you work with a singer and they mostly just do vocals, but occasionally they inspire you to write songs a certain way to suit they're vocals?

How would you handle it if you wrote a song and advertised for a singer online. Somebody answered your ad and did the vocals for you but you've never actually met them in person. Nothing was discussed as far as payment or anything. Then a year later, the song becomes a hit. How would you credit the person and would you owe them any money? If so, how much?