On the subscription tab you can see your current subscription status.
You can apply there the auto-assignment feature that allows handling user subscriptions.
When auto-assignment feature is enabled, any user who opens SharePoint Calendar add-in or opens the page with installed SharePoint Calendar add-in, will be added to licensed users list automatically. This feature can be a great help if your company has hundreds or thousands of users. SharePoint administrator does not need to add manually all licensed users to list.
Also, you can apply cleaning feature to automatically delete from licensed users list all users who have not opened SharePoint Calendar more than 30 days.
Another option is site collection url(s) assignment for adding licensed users. You can define the list of SharePoint sites where the users auto-assignment feature can be applied.
Now if a user opens Virto SharePoint Online Calendar on any site that is NOT contained in this list, he will not be added to licensed users list.