Jetpack’s Single Sign On feature lets your site’s visitors log into your site using their WordPress.com account.
To enable this feature, go to: Jetpack > Settings > Security and enable Single Sign On. There are a few options for this feature.
The “Match by Email” feature will allow people to login with their WordPress.com account and sync their WordPress account to their user account on your site based on matching emails. I would recommend enabling this feature, as it simplifies the process for existing users.
“Two Factor Authentication” sends someone a text message with a random string of numbers that they must enter in addition their username and password. It’s important to know that a user must enable two factor authentication in their WordPress.com account, which is not enabled by default. So, while this does increase security on your site, it also requires users to enable this feature in their WordPress.com account under security. If someone does not want to they will not be able to login to your site.
If you feel that integrating login to your site with WordPress.com would be helpful for you and your users go ahead and enable the Jetpack Single Sign On feature, then view the documentation page for the feature has additional information about the different settings possible.