They can do this in one of two ways:
1. You can contribute to a posting that someone else has already submitted and placed in the public gallery.
2. You can upload your own poem or short story and either place it in the public gallery or invite others to collaborate with you.
If the submissions are put in the public gallery, anyone can add to them. If you don't want anyone to be able to do that, then you can mark the poem as private and invite others to collaborate with you.
I would use this site in my classroom to foster student creativity. I would also use it as a tool to illustrate the significance of a worldwide audience.