If I might rephrase the question...
Your asking how do people access your server from the internet?
It is best to configure your server with a static IP address that will not conflict with the address that your router provides.
Next you will need to configure the router to foward FTP port (21) to the IP address of your server.
The clients will access the server via the IP address of the router or you can sign up for one of the free dynamic IP DNS services like www.noip.com.
That way you do not need to worry about when your router's external IP address changes.