I think that the key is not to set out to write a song, but to consider at an exercise to make you better at writing songs. We ALWAYS drink when we do this. We have a few basic rules: 1. Choose a key 2. Choose a topic (non-sequitors are always best, because these force you to think outside of the box) 3. Choose a tempo 4. You only get 4 counts at a time-period. 5. You are not allowed to suggest anything to anyone while they think. (you are allowed to drink and go outside and smoke)It's important to play with other songwriters who have a mutual respect for each other. And we are all folk musicians who play in the same area.
As far as taking credit, if you end up using a song you wrote in the studio or in performance, no matter how small the contribution, I always credit everyone. I don't care if someone suggested I use a C/G instead of a C, I still credit everyone involved as a co-writer. Anything less is just selfish.
It's funny, but it doesnt seem to work as well if you do entire verses because it gives the first person to go a chance to produce a whole mood and idea for the song. One line at a time ensures that no one can take over.
Again, it's primarily an exercise....