Hi,
The procedure for sharing a project is a bit involved.
Plug in a thumb drive formatted for Apple (Mac OS Extended, journaled). If it is not formatted for Apple you can format it using the Disk Utility app on your Mac. Be aware that reformatting the drive will erase all data on it, so be sure to move or copy onto a different drive any data that you want to save.
Create a new library on your computer and copy your project into it.
This link will explain how to work with multiple libraries:
http://help.apple.com/imovie/mac/10.1/#/mov3fa25bae7
Consolidate the library and the project to make sure that you are bringing bringing all data used in the project. This link will show you how.
http://help.apple.com/imovie/mac/10.1/#/mov882dee351
Then copy your library (Option-drag) to the thumb drive. Work on your project on the thumb drive. You can then pass back and forth the thumb drive with your friend, who can then work on the project and pass it back to you.
It will be necessary for each of you to have exactly the same version of iMovie on your computers, or it won't work.
-- Rich