This video tutorial goes through the steps to create a glass material in Rhino 7 using the PBR Material option. Timestamps to the key sections in this video can be found below:
0:25 - Creating a physically based material
0:40 - Base colour settings
1:07 - Opacity settings
1:45 - Specularity Settings
2:20 - Render preview
2:45 - Assigning to glass objects layer