Monitor if users accessed my Mac when sharing was open to all? I had opened sharing to everyone on my Mac as a "temporary" experiment to exchange files with another Mac. I forgot, and later that night when I went to shut down, the Mac reported that 2 users were connected. 

Is there a log to see who those users were? I assume one was me given my experiment. But the other?
The file sharing protocol AFP used by OS X does not log access per default. While (at least for OS X Server) reporting to a log file can be activated before starting file sharing, it is quite a "chatty" log process (meaning it generates huge, meaningless log files easily).

To put it simple: No, there's no way to find out who accesses your Mac beside yourself after file sharing ended.