For modern controllers, you can simplify this a bit. Instead of processRequest(), do:

public function handleRequest(AphrontRequest $request) {

Then do this to read the 'id' value:

$request->getURIData('id');

In modern code, prefer:

$viewer = $this->getViewer();

You can remove this, since we aren't requiring CAN_EDIT to join events (just CAN_VIEW).

Unused.

You can just omit this, the default button name is "Cancel" if you don't pass an alternate name.

Now unused.

Is there a better icon for this? fa-user-minus or something?

No need for $can_edit to join events.

As above.

If something passes a bad PHID, this could fatal. Maybe do this between these lines:

if (!$invited) {
  return blahblah::STATUS_UNINVITED;
}