Easiest way for the to communicate over ethernet would probably be AppleTalk. On each system, open the AppleTalk control panel, set the connection to ethernet, and have it switch the service on. If you have some folders or drives you want to make accessible, select them, choose "get info" in the file menu at the top of the screen, and choose the info "sharing" to set sharing and permissions. The activate the File Sharing control panel and start the file sharing service.
To find Macs on your network, go to the Apple menu in the top of the screen and select Network Browser. If everything is set up, you should see connected Macs listed there. You shouldn't need to make any additional accounts or change permissions if you log on as the owner/admin of each system.