Zim - A Desktop Wiki
Note that zim by design is a single-user program. So even though you could put a notebook on a shared drive and edit it with multiple people, this will not work very robust (e.g. two people can not edit the same page at the same time).
Therefore the best way to collaborate on a notebook is to put it under version control.
See the Bazaar user manual for some ways to use bazaar with different types of collaboration. Similar ways of working can be achieved with Git or Mercurial as well.
See the Version Control plugin for support in zim to commit revisions to a version control system.