iMovie "Join Clips" option greyed out
Note: Join clips option in iMovie cannot be used for joining two different imported video clips. It can only be used to join clips created by splitting an imported clip in iMovie.  

So, essentially if you have two clips which are to be combined, click on the Share icon in the Top right corner, then click on the option File and export all the clips as a single video file. You may then re-import it into the project if required after the share is successful. 

For a quick 2 min demo, watch this: https://www.youtube.com/watch?v=8Px_I5fvJbk