If an instance has no available license spots for <Application>, and the customer attempts to add <username> to a group that would result in <username> being granted <Application> access, they can click on the button to add the user / group however nothing happens as a result, and no message is displayed on the screen.

From the Javascript console, you'll see that the POST function that is failing. The response message says "<Application> - you have reached the license limit for this application. You only have 0 seat(s) left, but require 1 seat(s) to add these user(s): <username>."]}".

Error message in the console is:

POST https://instance.atlassian.net/admin/rest/um/1/user/group/direct?username=user 400 (Bad Request) jquery.min.js:2
Uncaught TypeError: Cannot read property 'errorBody' of undefined