In reply to Steve Wilkinson.

Each registered user on a WordPress site using Jetpack can go to the Jetpack menu and connect their account to their own WordPress.com account. There is no limit to the number of users that can connect.

The first admin connecting Jetpack to their WordPress.com account will be considered as the master Jetpack user, but all other admins who connected their own account will have the exact same privileges on WordPress.com. If someone can activate / deactivate / update plugins on the self-hosted site, they'll be able to do it on WordPress.com as well once they connected their account.

Once you've connected more than one account, you can monitor and manage the list of connected Jetpack users by going to the Jetpack menu in your dashboard, and clicking on "My Jetpack".