To join an existing Workgroup in Windows 10 do the following;

1. Navigate to Control Panel, System and Security and System
2. Find Workgroup and select Change settings
3. Select Change next to ‘To rename this computer or change its domain …’
4. Type in the name of the Workgroup you want to join and click OK
5. Reboot your computer for the changes to take effect
6. Navigate to Control Panel, Network and Internet and View network computers and devices to see other machines within that Workgroup.