How do I add / remove licenses for my business?

When you login head over into "Billing" or head over to the business overview page by clicking the Upvio logo in the top left.  Here you can view your available licenses.

Similarly "Addons" allow you to see what extras and fun features you can add to all your plans.
NOTE: Addons apply to all your businesses with active paid plans - you just buy it once and then all your businesses get them :)