I'm running LR 6.4 (stand alone) on Windows 10 64. In Library (any view) or in Develop, I can right click on any photo in the film strip and a menu shows with many options, including:
Go to Collection... (which shows collections this image is in),
Add to Quick Collection, and
Remove from Collection.
Also, if you have the collection list showing on the left, when you select a bunch of images from the film strip, you can drag the selected bunch to the collection you want to add them to.
Works for me.