Hi,
The classic way of sharing an iMovie file with collaborators is to place the iMovie project into an iMovie Library that will contain only that one project. Then transfer the iMovie library containing the project to a Thumb Drive that is formatted Mac OS Extended (Journaled) or APFS. You then pass back and forth the thumb drive between two Mac Computers. Both Macs must be running the exact same version of iMovie that was originally used to create the project. Here's a link that discusses the procedure in more detail:
Can you send/export an entire imovie proj… - Apple Community
Also, there is a possibility that you can use the Dropbox app to collaborate on an iMovie project. This involves transferring the iMovie library, containing just the one project, to Dropbox, where it can be accessed by others. I have never tried this myself so cannot personally verify that it works. Here's a link that discusses it:
https://www.picbackman.com/tips-tricks/how-to-backup-imovie-videos-to-dropbox-on-mac/
Always best to back up your library before moving it around.
-- Rich