A Collaborators Agreement is a fundamental part of any songwriters toolbox.
If you work with any other people when writing your songs, you need a Collaborators Agreement. Think of it as a pre-nuptial agreement for songwriters.
It spells out, in simple terms, what each artist contributed, what happens if one of the authors can't be found at a later date, and other terms like money spent on the project.
A collaborators agreement prevents future problems and will help to make your work more productive. You won't have to worry that "I wrote all the lyrics and half the melody, why are they getting half? Now I have to deal with telling them that." It will all be spelled out in the agreement.
How about 3 years later a publisher hears your awesome song and says, let me have it for my artist. You say "Great, but I can't find the co-writer." Guess what happens? The publisher says, "thanks, but no thanks." They don't want the headache.
A collaborators agreement will help you get on with the more fun aspects of the business; writing songs!
Here is a video showing how the process works if that is easier for you.