Participant commits to group
The following was contributed by anonymous trolls in 2004. Feel free to keep editing it.
In any web service or other creative or interactive effort, a participant commits to group review and use of something that is provided by that participant. Once that group is "done" with that material, the group commits to public use of some kind, even if it is a quite restricted notion of a "public".
There are other aspects of what a participant commits to a group beyond Common Content:
For instance there may be a nondisclosure agreement respecting the way the work was made, or a non-compete agreement, or a general understanding to act in accordance with the interests of all the group's members. A guild license for instance would probably require all of these things and allow for recourse in case of any non-compliance by the participant. There are also of course corporate licenses or outright releases of all rights including moral rights.