The private folders (/keybase/private/yourname), on the other hand, are end-to-end encrypted. That means Keybase's server will not be able to read any data you save in it. If you want someone else to be able to access it, you need to add their username. For example, if you want to share a folder with me, you'll have to create a folder like so: /keybase/private/yourname,mariella.
In the future, the team says you'll be able to share data even with people who haven't joined Keybase yet, so long as they have a Twitter account. Simply make a folder with your name and your friend's Twitter handle like this: /keybase/private/yourname,mariella_moon@twitter. The other person will be able to access those files as soon as he/she joins the service.
Keybase co-creator Chris Coyne says they're adding that feature, because they believe you should be able to privately share (encrypted) files with anyone without asking for more identifying info. We imagine it could be used to share scoop with journalists or to privately share data with people you've only just met on the internet. Coyne's blog post has more technical details about how the system works. If you'd like to give it a shot, scroll down to the "Getting access" section to sign up as an alpha tester, though you'd have to wait till you get an invite via email before you can try it out.