How can I review custom privileges?
What I found out is that the "Custom" privilege means that permissions have been configured in a way that are more complex than is provided and can be shown in the basic user settings of the Get Info window (namely: Read, Write, No Access).

If I query the volume in Terminal with ls -leda /Volumes/Some\ HD/ I get something like this.  Option e is to print the ACL associated with the file (or directory), if present, in long (-l) output.

drwx------+ 18 Alex  staff  680  1 Oct 18:19 /Volumes/Some\ HD/
 0: group:com.apple.sharepoint.group.6 allow search
 1: group:com.apple.sharepoint.group.5 allow search
 2: D93ABBA3-3566-4A08-878E-B2E7C575B4A6 allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity


I just still don't know where the com.apple.sharepoint.group users are coming from and D93ABBA3-3566-4A08-878E-B2E7C575B4A6 looks a bit cryptic if that's me.