From the Tools menu, select Compare Texts… .
It looks this this has changed to:
From the Project menu, select Compare Versions… .
Still, I wish there was a more direct way of selecting a user, then a time window, and seeing that user’s contributions to the project.
In my case it has recently come to my attention that some users have edit access that I hadn’t been aware of. I’m not sure since when, or when they might have made edits.