workgroup in windows has zero to do with shared resources. All a "workgroup" is what list to put it in the browselist, nothing more - it really is just utterly useless.. Might of be somewhat useful back in the day of windows for workgroups to group machines in specific lists like users, servers, etc.. dept A, dept B, etc..
But they have zero to do with authentication to who can access shared resources off a machine.
Might was just leave it at the default workgroup for the name. The DNS domain that you setup in pfsense is just that the domain part of a FQDN (fully qualified domain name) so when you lookup pfsense.something.tld or whatever you want to call pfsense. Or other machines in your network.