If I run my own minecraft server, will I have to pay for the clients?
Just get this one out of the way but I think you already know, the PC and XBox versions of the game are not compatible.
As for the simplified question, Yes. You should buy an account for each user to connect to the server. The server itself does not need an account and is a separate download from the client software though it is possible to 'Open to LAN' a single player world.