Group commits to public
The following was contributed by anonymous trolls in 2004. Feel free to keep editing it.
What a group commits to public might vary drastically from what each participant commits to group use. For one thing, not all tools and methods used in the process of creating a work are necessarily visible as the output: what is published might be only a tiny fraction of what was written, factored and reviewed in private. The group might be in possession of some skill or facility that it wishes to retain as an advantage in the marketplace, and may wish to allow only some temporary use of this facility to participants who might be restricted from copying it elsewhere. Finally, the group may only be possible to form or sustain if it has a commercial interest separate from that of the participants in the value of the works it facilitates. For all these reasons there may be a need for a variance in the commitments on the inputs and those on the visible output.
A Share Alike license might still be applied by a group to its public works even if it has severe restrictions and obligations - but it probably cannot offer the same obligations to every participant, e.g. the Green Party of Canada Living Platform Terms of Use which distinguish carefully between "participant to party" obligations (under CC-by) and party to public obligations (under CC-by-nc-sa).