In HOC 3.0 -- A contact's Main Organization is the one that corresponds with the Organization Name in their contact record.  They are granted partner staff access by clicking on "Change Profile" and at that time they have access to the sharing portal to manage that organization.

In the screenshot below, Art Trout has partner staff access for the Organization Troutco.

In the new related list "related organizations' in the contact record, we see that Art Trout has a relationship with the organization Troutco.  The checkmark next to "Direct" tells us this is his main organization  (the one he is an employee at generally).  He already has partner staff access to this organization because its his principal organization and he has the profile partner staff.

You use the related organizations related list to add additional organizations that Art can access and manage in the sharing portal.  Here's how to do this:

1) Click on the  "Add Relationship" button.  This will open a New Organization Contact Relationship record

2)  Choose another active partner organization you wish Art to be able to manage.

3)  Set the value of HOC Access to "Partner Staff"

4)  Save the Record.

Note:  The field 'roles' is entirely optional - but can be customized and used to keep track of the relationship detail that this contact has with this other organization.  In this example we have customized the picklist values in the roles field and added the value "Opportunity Coordinator" and "Primary Contact" - so we will keep track of the fact that Art's relationship to Xavier's School is he is an opportunity coordinator for them.  .

The related list will now show two organizations that Art can manage:

He is a direct contact for Troutco  (it isn't necessary that he be granted HOC Access for his main organization), but he has also been granted Access as Partner Staff to "Xaviers School...."     He is now able to manage both organizations.  In the next step we'll show you how!

When Art logs in to the sharing portal, he's given a welcome screen which allows him to choose which organization he wishes to manage.

He can choose either organization:

Once logged into an organization - he is able to manage that organization.  Only that one organizations volunteer opportunities, connections, contacts etc will be accessible to him at one time.

He can see which organization he is currently managing on the left sidebar.  The organization logo also appears:

To switch organizations, he just needs to click on the picklist dropdown to the right of the organization name. Doing so will reopen the organization selector screen and he can quickly switch to any other organization he has access to manage, without logging out and starting over.