That isn't the issue. The issue is YOU being able to share MY WiFi key because I was dumb enough to let a Windows 10 user on my WiFi network. This is akin to me giving you the keys to my house so you can housesit, and you getting a hundred copies cut and distributing them to a bunch of people you know.
If you *tell* someone your WiFi password *then* there's nothing stopping them from sharing it with whomever they want. So do not do that. Not if he brings OS X or Linux or Windows.
If you want to allow some friend onto your network but not allow him to share your network with others, then *you* tap in the password at his computer when it connects. On OS X or Linux or Windows. That what you would do today, and that's what you would do when your friends brings a Windows 10. On Windows 10 simply DO NOT CHECK the "share" checkbox. It is off by default. Your network will not be shared.
Nothing has changed. Neither your network nor your password will be shared with anyone. Your friend cannot go into settings and share the network after the fact - it has to be done when connecting.
But if *you* connect to some network which you would like to share with your friends, you can check the "share" checkbox. When you do that, your password will be stored encrypted in Microsofts servers. When one of your friends (if you share with - say - Facebook friends) is in range of that network, his Windows 10 computer can engage the network. The network will issue a challenge with must be hashed using the password as salt, and the hash returned. Modern password auth works like that to avoid sending passwords in cleartext. This means that the *actual* password hash is a one-time hash computed from the challenge.
The computation of the hash is performed on Microsofts servers, and your actual password is NEVER available on your friends computer - not even in encrypted form - only the challenge response hash. Your friends computer must obtain the response to the challenge from Microsofts servers - and when doing so it must prove that it belongs to a friend of yours.
Furthermore, Windows 10 which connects to a network in this way will *not* allow access to other devices on the network except for the internet gateway. I.e. it can only be used for Internet access - nor for local file or media sharing.