BigSheets doesn't have any user-friendly way to embed a workbook in any other context. The BigSheets page is actually its own iframe, which includes the full BigInsights banner, so if you wanted to embed only the sheet itself that would be quite difficult.
Using the BigInsights authentication API it may, however, be possible to embed chart results placed on the BigInsights dashboard. I do think these steps will work:
1) Create and run a chart off of a workbook. Give the chart a name: for example "mychart"
2) Now go to the Dashboard tab in BigInsights. Create a New Dashboard: give a name like "mydashboard"
3) On the dashboard click "Add Widget"
4) In the Add Widget dialog select your chart, mychart, and place it on the dashboard.
5) Save the dashboard.
6) After saving the dashboard you should now see a link icon above the chart.
7) You can now directly reference this chart from the outside world as long as you first authenticate to the cluster. If you were creating a webpage to display the chart you could code your page to authenticate to BigInsights using the authentication API and if successful then chain the success response to then make a call to the chart link.