The Delve application itself is not available for external users, but the signals from external users are. I have previously created a Delve clone using the Content Search Web Part, which also works for external users. It should be polished a bit though :)
This time I decided to go the easier route to avoid doing any polishing.
- Navigate to Delve from a random tenant you have access to https://delve.sharepoint.com/_layouts/15/me.aspx
- Do view source in the browser and save the markup as delve-rip.aspx
- Navigate to the site pages library in a team site
- Upload the file saved in step 2
- Share the site with an external user
- Navigate to the shared site and page
Want to know more… give me a shoutout!