Someone in chat suggested to me that since the sandbox front page has this text: http://scp-sandbox-3.wikidot.com/
Currently, only page creators and mods/admins can edit pages. This prevents people from editing sandboxes that are not theirs. For more information on sandbox permissions and how to share sandboxes for co-authored works, please contact Zyn or another member of site staff.
That a link to the page I wrote about co-authoring could be added: http://www.scp-wiki.net/zyns-co-authoring-tips
You can also refer to this page on co-authoring.
I added the link. Feel free to let me know if there are any objections/suggestions on an alternative action.