Server without a Server?

Help...

I just started work in a 4-person office. We really (really) need a simple way to access shared files like lists that keep being added to. I don't know if we needed a server program or if it can be done in system preferences.

Anyway, I can do "connect to server" to get to the other computers but I really just want 1 folder (or user account) on 1 computer that holds all the shared stuff. Something that comes up on each computer's desktop when you start up (or have to connect to but isn't too complicated).

3 macs are connected to the network by ethernet. 1 by airport. We're using osx 10.39. I don't know much about networking, just looking for a general recommended approach, or step-by-step, or link to an article.

Thanks...