Selective file sharing in a public coffee shop
You can share a folder by selecting it in the Finder, press CommandI (Or select Get Info), and check the box that says "Shared Folder". You will need to enable file sharing in System Preferences if you have not already done that. 

Your second and third questions have a single solution that could solve both problems. Unless the users have accounts on your machine, you are correct that you would be making the files available to anyone on the network. However, you could create a peer-to-peer network that only your friends have the password to join. Go to the WiFi menu and select "Create Network." Assign your network a name and password, and give that information to your friends.

Be sure to disable sharing when you are done, or else members of other, more public networks will be able to read your files.