First, a word about sharing ownership.  When you own something in Plone, it does not have to mean you are the sole owner of it.  Quite often you'll share ownership with someone else.  What this means is that having ownership of something allows you more privileges for modifying it.  You must be given ownership by a manager who approves of such things for the site.  Below explains how objects of the site can have their ownership shared by a user and a manager.

Let's say you've got a role as Manager for your site and you need to have a user of your web site be able to view and edit particular pages or folders of your web site (or several users viewing an editing several other web site pages or folders).  You will likely need to be able to review their edits and approve them for publishing.

This requires the user or users being added to the web site and having the Reviewer role (or a role that you can create and configure, though Reviewer is fine for the above purpose).  If the user does not already have an account in the web site, create one by going to Site Setup, Users and Groups Administration, where you can add a new user. 

Click on Add New User, fill out the form presented, click the Save button, and you're then presented with the screen showing the new user has been added, in this example, Dave Reed.  On this screen, give him the Role of Reviewer and give him access for him to change his own password (that way you  don't have to worry about it) by clicking those checkboxes.

The role of Reviewer gives him the access to view and edit the pages and folders he is given shared ownership of.

To give Dave shared ownership of a page or folder, go to that page or folder, and in this example its the within Resources folder. 

We'll want Dave to have editing capabilities for the Cars folder and the Index page of the Resources folder.  From the Resources folder Contents tab, select the page titled Index, and select the Sharing tab on the Index page.  Here you'll see a list of users of the site.  In this example, gates2 is the only one who has the local role of Owner, which means gates2 is the only one who can edit this page right now.  We will add Dave Reed as an Owner too.

Go to the " Add sharing permissions to users" section and type in the user's name, in the example dave.

The result brings up all daves, in this example there is just one, and its listed as Available Members.  From the "Role to assign" drop down menu, when dave is found check the checkbox to the left of his name, select Owner, and click the "Assign Local Role to Selected User(s)" button.

Dave, the user dreed, now shares the Role of Owner for this one page, Index, with gates2.  Once this is done, gates2 and dreed are allowed to edit this one page (gates2, as manager, has editing access to the entire site as well).

Bear in mind that when Dave was given shared ownership of the page Index, an Attention box at the top of the page pointed out that you could change sharing permissions for the container that the page Index was in.  This is not what we need to do as the page Index was the one we want Dave to share ownership with.

Moving on the Cars folder, select its Sharing tab, and you'll see Dave does not share Ownership here yet. 

Do the same as you did for the Index page, search for dave, when dave is found check the checkbox to the left of his name, select Owner from the "Role to assign" drop down, and add dreed to those who share ownership of this folder.  This will provide editing access for all the contents of this folder.

With these permissions that the Role of Owner gives users as they share editing capabilities of folders and pages, collaboration exists between writers and editors of content and those who approve the content for publishing.