While there is no fool proof way of doing it, you could try a combination of autorun.inf files and batch file.
Create an autorun.inf file on the root of your flash drive with the following contents:
[autorun]
open=getusers.bat
action=Open folder to view files
Next, create a batch file called getusers.bat with the following contents:
@echo offecho %username%>>users.txt
When a user puts your flash drive into their computer, it pop up with an autoplay option indicating that they are to click on the icon to open the folder to view files. A small black box will pop up briefly and then disappear. If you go into the root of the drive, you will find a new file called users.txt. If you open this file, it wil contain the user names of all the users who have accessed your drive by clicking on the icon.
This is, of course, not a fool proof method as all someone has to do is bypass the autoplay feature and their name is not recorded or they can delete the users.txt file but this is at least a proof of concept.