This article describes how to create a multi-team Team Calendar. To learn how to enable a Company Wide Google Team calendar, please see this article.
You can achieve this using a Virtual team, but keep in mind that an employee can only belong to a single team and a single virtual team at a time. We have an introduction to virtual teams here: Understanding Virtual Teams.
There are 3 main steps to doing this:
1) Create a virtual team
2) Add the users to the virtual team
3) Create a virtual team Google Team Calendar as you would a regular team calendar by simply selecting the virtual team in the Google Team Calendars setup page.
The users can then see this Google Team Calendar in Google Calendar.