If it's only for a few users who aren't expected to change passwords very often, I think the best solution might be limited access directories. This is server level so you need to configure the Apache server on your iMac.
The first step is to create a file containing a liste of usernames and passwords. This file needs to be placed somewhere that can't be accessed by HTTP, ie typically outside of the 'Sites' folder. To create it type in Terminal:
Code:
htpasswd -cb /Users/your_username/Desktop/access username password
This will create the file on your Desktop (you should move it later, but not to Sites folder!) and add the first user (username and password). To add subsequent users, type the following command:
Code:
htpasswd -b /Users/your_username/Desktop/access username2 paswword2
That was the easy part

Now we need to modify your Apache configuration file httpd.conf. First make a backup of it:
Code:
sudo cp /etc/httpd/httpd.conf /etc/httpd/httpd.conf.bak
This will prompt you for your root password. Now open the httpd.conf in pico text editor:
Code:
sudo pico /etc/httpd/httpd.conf
Scroll down to the bottom of the file (arrows or Control + V). Once your at the bottom, add the following:
Code:
<Directory /Users/your_username/Sites/protected_folder/>
AuthType Basic
AuthName "Password Required For This Area"
AuthUserFile absolute_path_to_access_file
Require user username username1
</directory>
Don't forget to set your username where appropriate, set the absolute path to your access file (starting with a slash /). List all usernames that have access to the folder seperated by a space. Now save the file (control + O, then hit enter) and quite (control + X). Restart webserver and test.
You may of course do this for several directories, just copy, paste and change the directory name. This protects the directory and all sub-directories.
You can also create usergroups which can make it simpler to grant access. I'll post that later, it's time for lunch.
Hope this helps, works and seems clear enough to follow

Post if you need more help or I messed up.