(1) If you are going to write a song with others, please be encouraged to do so with the most generous spirit possible!!
(2) It is always going to be wisest to determine the protocol for writer splits before you start writing.
If you start a song from scratch together, agree to split the song evenly between the writers - irrespective of who contributes what.
If one of the writers comes into the room with a developed melody and/or lyric, then tilt the splits in his or her favor. 75/25 for melody and 75/25 for lyric. But do this upfront, before anyone plays a note of music. Otherwise, PLEASE stick to the Nashville protocol of equal splits and determine to communicate better upfront next time.
Look for ways to do good to contributors. For example, if one of your instrumentalists comes up with a riff that becomes inextricably integrated into the song, then leap at the opportunity to cut him or her into the writer splits.
Discuss the idea of a lyric doctor upfront - see blog titled A Message To Songwriters - and predetermine the percentage that could be given up to a lyric doctor, and who this could be. If the song was inspired by a sermon, consider cutting the preached into the deal. For example, The Kingdom Is Yours was inspired by a talk by Aaron Keyes. Dee Wilson, Brittney Spencer and Micah Massey welcomed Aaron into the arrangement as a 10% writer.
(3) After you have finished your first co-writing session, no-one should add in another writer before consulting the existing co-writers.