As explained in an FAQ for the new feature, devices running Android Nougat can act as hosts for the connections, however older devices running Marshmallow can only use the shared data connections.
In order to use instant tethering, you’ll also need to make sure that any devices you want to connect to the network share the same Google account.
Then, head to the Settings menu on your device, then head to Personal.
Then, tap Google Instant Tethering, and you’ll be able to select Provide Data Connection and set up a host device.
Once this is set up, follows the same steps but select Get Data Connection to set up a non-host phone or tablet.
From then on, these secondary devices should automatically connect when they detect the host device is nearby.