Yes, you can add more users at any time. Please, return to the site you have purchased the fonts and proceed as follows.
How to add users to your Desktop license? The simplest way is to create a new online order with a new font license with the desired number of users.
How to increase the Webfont license? For WebFonts Licenses, simply create a new font order and only purchase the supplement for number of page views per month. For example, if you first purchased a license with 10,000 views per month and your site has increased its attendance with over 20,000 page views per month, create an order for only the additional 10,000 views.