Search the online help for "Work with multiple libraries".
What I did was (copied from the help):
Create a new library
Choose File > Open Library > New.
In the Save dialog that appears, type a name for the new library, and navigate to the location where you want to save it.
The default location is the Movies folder in your home folder.
Click Save.
A new library is created in the location you chose, and a new, empty event with today’s date is created.
Then copied the projects to the new library:
Copy or move projects between libraries
Connect a storage device that contains the target library to your Mac, or copy the target library to your Mac.
Choose File > Open Library, and choose an option from the submenu.
You can choose from recently opened libraries, locate an existing library on your Mac, or create a new library.
The selected library is opened in the Libraries list, with the first event selected and its contents displayed in the browser.
Click the Projects button in the toolbar.
* Select the project you want to move or copy.
* Click the button to the right of the project name, then do one of the following:
* Copy the project to another library: Click Copy to Library, then choose a library from the submenu.
* Move the project to another library: Click Move to Library, then choose a library from the submenu.