Ah, didn't know about PureFTPd. Looks fantastic.
So you can use PureFTPd as the daemon in each machine to allow the other machines to connect to it, and Transmit as the client in each machine to connect to them.
If your friends want a cheaper FTP clients, check out
CyberDuck, which I mentioned above - it's free.
Also, to be able to connect from one computer to another by FTP, you'd generally require a static IP, an address uniquely identifying your computer on the Internet. In all probability, your IP address changes every time you login to your network (this is dynamic IP).
If you have static IP, great! -- Just connect using your static IP as the server on port 21, enter credentials you created in PureFTPd, and you're logged in.
If you have a dynamic IP, there are things you could do.
a) Find out your new IP address everytime you connect using
http://www.whatismyip.com/ and then connect to that server. So if you wanted to connect to your friend's computer, you'd have to ask her to do this on her computer.
b) Use a service like No-IP (
http://www.no-ip.com/services/managed_dns/free_dynamic_dns.html) or DynDNS (tutorial here:
http://www.macinstruct.com/node/152) to get a permanent address (a subdomain) that is always updated to forward to your active IP address. This makes it easier for everybody.
There might be one more problem if you're behind a router: whatever your external IP address is, it'll be that of the router and not of your computer. What do you do?
You can do some port forwarding on your router (tutorials for doing this to work with No-IP here:
http://www.no-ip.com/support/guides/routers/), or put your Mac in DMZ (Demilitarized Zone), the second of which I don't understand very much at all.
So setting up an FTP machine to be accessible from the public Internet can be very difficult indeed.
You can also connect directly to another Mac if you know the IP address (using above methods). In Finder, go to Go > Connect to Server. In the dialog that appears, just enter the IP and, if the other Mac has public sharing enabled, that's it!
(Yeah, it's not very easy, but when it's remote networks, unique IP addresses come into play).
Hope this helps.