To add or change your subscription, follow these steps:
From the account menu in the upper-right corner of the screen, select Subscription & Billing
A User Subscription and bills page will get open which will show you all your billing records.
Now if you want to increase the number of user, click on drop down menu select the number of user and click on Change Subscription.
You can use the selected number of users now onwards. After some business days you can pay for the increased number of user and enjoy the application for more users.