Unfortunately there is no way to link free accounts/ premium accounts. Each person would have to sign-up through their own email (the same email cannot be used for multiple accounts).
As for the premium subscription, it would have to be made through the mobile app because the payments are made to the Play Store or App Store depending on your phone type.
There is also the option to create an account/ log into an existing account at www.textnow.com/login through your web browser if you wanted to use it over your computer instead of the app.
For further assistance please contact our Live Chat between 10am-5:30pm EST