In Apple’s continued drive to simplify macOS Server, in High Sierra they have completely removed the File Sharing section from the Server.app and force you to use the basic File Sharing settings in System Preferences > Sharing.

Whilst you can do most of the things you need to in System Preferences now, you still have to go over to the Server app to properly set permissions on the folder hierarchy and if you want to view (and disconnect) connected file sharing users, then you’re out of luck unless you drop into the Terminal

Alex Narvey from Precursor Systems has helpfully created the FileSharing Assistant for macOS Server 5.4.

FileSharing Assistant is a FileMaker Runtime solution (that doesn’t require FileMaker Pro to be installed) that uses command-line tools installed with macOS Server to list connected users and gives you the option to disconnect them.