Wi-Fi Sense does two things: It lets you share your Wi-Fi network's login credentials with your contacts, and it lets you connect to networks shared by your contacts. This can be useful if you frequently change the password to a network that's used by a large number of people (though enterprise networks using 802.1X can't be shared), or if you want to allow people to access the Internet without giving up your password -- assuming all of those people have Windows 10 and have Wi-Fi Sense enabled. Here's how to make it happen:

Turn Wi-Fi Sense on

If you set up your computer using Windows 10's Express settings, Wi-Fi Sense is turned on by default. This means that you'll be able to see and connect to any Wi-Fi networks shared by your friends; it does not mean that Wi-Fi networks you connect to will be shared by default.

If you didn't set up your computer using Express settings, you can turn Wi-Fi Sense on in the Settings menu. Go to Settings > Network & Internet > Wi-Fi and click Manage Wi-Fi Settings.

To turn Wi-Fi Sense on, toggle Connect to networks shared by my contacts to on. Here, you can also set up which of your contacts will receive any Wi-Fi info you decide to share, by checking (or un-checking) the boxes next to Outlook.com contacts, Skype contacts, and Facebook friends.
If you choose to share networks with Facebook friends, you will need to click Wi-Fi Sense needs permission to use your Facebook account and log in to your Facebook account to give Windows permission.

Share a network

Even if you have Wi-Fi Sense turned on, networks you connect to are not automatically shared with your contacts. To share a network, you'll need to go into the Wi-Fi Sense menu and manually share it - by retyping the password.

To do this, go to Settings > Network & Internet > Wi-Fi and click Manage Wi-Fi Settings. At the bottom of the page, you'll see a section called Manage known networks. In this section, you'll see all your 'known' Wi-Fi networks - networks you've connected to in the past.
To share a specific network with your contacts, click the name of the network and then click Share.

You will need to type in the network's password and click Share again. Once you do this, that network's log-in information will be shared with your contacts in encrypted form -- nobody will see the password, but they will be able to automatically connect to that Wi-Fi network if they are in range.