You first need to confirm if the builder group has been added to Streetscape. You can do this by going into your Inventory Setup (formerly Community Settings) and clicking the Edit button on the Community you want to add them to. 

Once in the Community Information, select the Builders tab in the black toolbar at the top of the page and select the Add Builder button; 

You can search in the dropdown for the builder group name, if the group exists, highlight the name and click the Add button.
If they don't appear in the list, click the "invite them" link to the right hand side in purple. This will take you through the process to set up a new builder group and add users.