Can a Mac join a Windows Homegroup? Workgroup is his "domain" name though it really isn't a domain server.

Is this a wireless printer or is it connected to one of the Windows PCs? If it is a wireless printer this shouldn't even be happening. If it is connected by USB to a Windows 10 computer, then that computer is functioning as a print server, and it would more than likely be asking for your Windows un/pw so it knows Fivestring is a valid user on that computer.