It might be easier to get the ball rolling if there was a definitive starting element, and you identify different parts of the song where folks can contribute.
First, provide an initial seed to inspire and kick off the process. It could be a riff, chord progression, lyric fragment, song title, an 8-bar loop, etc... something that acts as the kernel that fuels the rest of the songwriting effort. And it would also help to identify a musical genre at the outset.
Then identify some areas for contributions. Perhaps someone could provide a chord progression, another person might come up with drum beat, another could come up with a bass line, someone could add backing instruments, someone could come up with a signature riff, someone could come up with a melody, and someone could write lyrics, and of course, someone needs to record vocals, etc...
The cool thing about songwriting, is that a song is made up of multiple components that can be taken separately, so everyone can work on specific elements that all fit together in the end.
Now for the thorny part - rights and ownership. This is going to get dark...
What if five forum members make contributions to a song, and by sheer chance, the song gets picked up and used in a TV commercial that pays $30,000. That may sound far-fetched, but it's totally within the realm of possibility. It happens.
Who gets paid? Who owns the publishing? Do all contributors get paid equally? Does someone who made one comment in the thread suggesting a chord should be changed from major to minor, deserve a share? Does someone who pointed out a typo in the lyrics deserve a share? Who decides who deserves a share? And what if someone feels they deserve a share, but aren't included? What if one of the contributors wants to self-publish the song, do they need to get permission from the other contributors?
These are the kinds of sticky issues that can arise when creating something collaboratively that might have commercial potential, especially on a public platform with strangers.
@David Cuny started a thread last year in which he bravely shared a song he was in the process of writing, seeking comments and input. He clearly stated at the top of the thread that it was entirely his song, and he was simply looking for feedback. In other words, if he adopted people's suggestions, he would still be the only writer of record - there would be no co-writers.
So if people are going to be willing to collaborate in this kind of environment, ground rules need to be established and clearly stated up front. It's not an easy thing to do, because there are lots of gray areas and potential loopholes that need to be taken into account.
Perhaps the simplest thing would be to clearly state that whatever gets created here is under a Creative Commons license. But what if down the road, someone violates that agreement and effectively "steals" the song for themselves - who is going to take action, and what kind of action will that be?
Sorry to get all lawyer-ee about this, but as someone who makes music for money, these are the kinds of issues that, if not clearly dealt with up front, can turn a fun creative project into a icky nightmare of resentment, bad feelings, and potential legal hassles.