Thanks for clarifying the question Marimba, I'm revising the answer to better address the use case.
The use case seems to be the following:
As a user I want to search for classes taught by either Greg Anderson or Julia Brown. Now I want to easily modify my search to also view classes offered by House.
So the system definitely needs to accommodate for "OR" type searches. On the plus side, it seems like the user does not need the flexibility of both "AND" and "OR" searches used together. Supporting both types leads to increase complexity as I've mentioned in the old answer and should be avoided whenever possible.
A good way to illustrate this is with checkboxes on faceted search. Checkboxes makes it easy for users to understand they're allow to select multiple. The number of items shown next to the facet options makes it easy for users to tell that they're adding to the results lists by checking more options. Keep in mind you should always display near the top of the search results how the filtering is done so it's easy for the users to understand what's going on.
download bmml source – Wireframes created with Balsamiq Mockups
If there are too many items to display them all within the sidebar, consider only showing the top matches, with a link to pull up all options. Or provide a "autosuggestion" type search mechanism. See below for example.
download bmml source
I'm going to have to ask the question, why? What is it that your users are trying to do?
Most search and filtering mechanism operates on the ability to refine your search results by providing more information. It's not a good idea to break this mental modal.
That said, there are systems that allow you to add to your search scope. They tend to be limited to use by power user. Sometimes system provide a text based "query-like" input, other times they are like mail rule builders to track which criteria will narrow search and which will widen search. Both approaches are not easy for a typical user to understand. So unless your users really need it, don't. It's better for users to perform 2 separate searches instead.