I thought I should further make it explicit that if you end up in the situation where you are unable to license the wiki because it was left too late - not only could users be issued take-down notices by other users, but you could be issued take-down notices by your users. As far as I know, any copyright holder (i.e. contributor) can walk away and demand the removal of their contributions, and potentially, any derivative works; i.e. subsequent edits. Thorny, thorny stuff.