Where should I place shared folders on an OS X Server? What is the appropriate/reccomended location in the drive hierarchy to place the folders shared by the users accessing the server (as fileserver) ?

Is it in /Groups /Users/Shared or ~/Documents/ ?

Or should I create a separate folder in /Users for each group ?

Is there a document/webpage that supplies instructions about that ?
Generally, the shared folders for a group should go in /Groups/groupname/.
/Users/Shared/ is more designed for local users to share, though there's nothing stopping you from using this too other than it is non-standard.