Protagonize
Originally for developing collaborative works, Protagonize has been recently redesigned and revamped completely and allows all of the things you're looking for.
Protagonize now has nearly 21,000 members and encourages interaction between authors and readers, allows you to build an ongoing and engaged readership, create and participate in writing groups, and collaborate with other authors on writing projects.
We consider the site a writers' playground; a way to flex your creative muscle and showcase your writing. We've also had a number of our authors get published using their work on the site as a reference or portfolio. We also give authors full control over licensing of their publications, and fall back to a Creative Commons Attribution-NonCommercial-ShareAlike license for all collaborative works.
Disclaimer: I'm the developer/operator of Protagonize, so feel free to ask me any questions about the site or how it works.