There are a couple of different ways in which you can add new subscriptions to your account. One of those ways would be through your account dashboard and subscriptions section. The other would be directly from the respective product page itself.
From your main account dashboard click on Subscriptions from either the main section or the sidebar on the right.
Once under your Subscriptions, you will see a box just below +Subscribe To More Content. From here you can type in and search for content or click on the arrow to have a drop down box displayed.
From the drop-down, select the subscription you would like. Reminder, you can scroll as well as search by typing in the name. In this example, we will select Top News-in-Pictures.
Once selected, you will see the subscription in addition to its single license cost. Increase the number of licenses as needed for your number of players, the total will adjust and calculate accordingly. You will be presented with a review of your current monthly costs as well as what the additional subscription will bring your new monthly total. Lastly, it will display the prorated charges for today. When ready, click Save Changes.
After saving your changes, you will get a confirmation as seen below.