Without Active Directory integration, Sharity cannot display correct file ownership and groups. Instead, it always returns the user-ID of the process accessing the file as owner of the file.
If you sometimes see "nobody" as owner, the file is probably written or read by a daemon which runs as "nobody" or at least performs this operation as user "nobody".
If you need real ownership, you must enable the option "Convert Windows ACLs" in the "Options" section of Sharity's GUI. This option requires some kind of domain integration, though, so that server and client have the same concept of users and groups.
There are various ways to achieve domain integration. See Sharity's manual, especially the section about Unix Home Directories on SMB Shares.