An iMovie Event or iMovie Project is tied to one user account at a time. It must be in your User/Movies/iMovie Projects or User/iMovie/iMovie Events folders respectively, and these folders cannot be accessed by someone else on your same Mac.
So the easy way around it is to place the Projects and Events on an external drive.
The external drive should be formatted as Mac OS Extended (journaled). If you have a drive that is formatted as FAT32, you can reformat it using Disk Utility.
Performance will be best using a Thunderbolt or Firewire 800 drive, but a USB drive will work too for everything except importing directly from a DV Tape camcorder (you must have Firewire to import from DV Tape).
To move your Projects and Events to the external drive, you must do the moves from within iMovie.
The best way is to click VIEW/EVENTS BY DISK. This will show you all available disks.
Hold down the Command Key as you drag your Project(s) to the external drive. You should be in the iMovie Project Library View. You can drag the small icon for the project to the small icon for the external drive, while holding down the command key. (Note: if you do not hold down the command key, it will copy, not move. This is OK, but will not free up any space on your internal drive, and may confuse you as to which copy you are working with. But you can come back and delete it later).
Now, you will want to move all the other project assets, like the Event Clips, the Photos, and the Music so that it is also available on the external drive. To do this, select the Project in the Project Library and click FILE/CONSOLIDATE MEDIA...
This will prompt you to move the Events over and copy the music and photos over.
Now you can both work on the projects.
