Based on your requirements that no one else be able to access the share, and that you don't want to hand out passwords, I'm going to suggest you go outside the OS and use third party software for this.
It happens to cover this use case in a nice way that gives you the security you want and the convenience you're after. It's also OS-independent which is nice if you're friends don't also have Macs.
- Download and install Dropbox on your Mac. Create your account and get it up and running.
- Under your new
~/Dropbox folder create a new folder. Call it something like
Shared with Bob so you know anything you put in it will be shared with Bob.
- Log in to the Dropbox web console at http://www.dropbox.com
- Find the folder in the web console. Click once on it to bring up the folder options menu and select 'Invite to Folder' from the list of options.
- Put in Bob's email address.
Once Bob joins Dropbox and the folder as a collaborator, anything either of you put in this folder will be sync'ed to the other person's Dropbox account and any computers they have linked to that account.
Once you're done sharing, reverse the process:
- Find the folder in the Dropbox web console.
- Click on it to bring up the options menu.
- Select 'Shared Folder Options'
- Click the 'Members' tab, find Bob in the list and say 'Kick Out'
You'll be given the option to let him keep the files on his disk or have the wiped. Up to you what you do.
With the slight inconvenience that the data you want to share has to live in your
~/Dropbox folder, this covers all your requests: it's secure (only you and whomever you invite to the folder can see the share), it doesn't require any passwords to be handed out, you don't have to enable any services on your Mac that might give other people on that public WiFi access to your machine.