Connecting a PC that is part of a Workgroup and therefore not the domain to the domain is a simple and easy to do.
On both the Workgroup and the Domain PC you want to open the "Network and Sharing Center" and go to "Advanced Sharing Settings" and make sure that on both machines the "Turn on file and printer sharing" has been selected.
Workroup to Domain
On the Workgroup PC in this example called PublicPC you can open file explorer and in the URL field put:
\\domainpc\C$
This will prompt for credentials which you will input as such:
Username: contoso.com\user1
Password: The correct user1 domain password
In this example the C drive has already been shared as C$
Domain to Workgroup
We can also do this in the reverse from the domain PC to the workgroup PC.
On the DomainPC open file explorer and in the URL field put:
\\publicpc\C$
This will prompt for credentials which you will input as such:
Username: publicpc\user1
Password: The correct local user1 password on the publicpc computer
In this example the C drive has already been shared as C$