There is no black and white answer to this I'm afraid. It's down to the companies discretion. Typically upper management or line management will make the decision but will seek the advice of someone within the organisation that is familiar with some aspect of the agile methodology

For me when it comes to UX inputs from experts are priceless. If ever I am working on a project or product I am unfamiliar with I would always try and bring in an "expert" or someone who uses frequently. If you're a reader, read the book "sprint" there are some amazing insights as to the value of experts and why there is no shame in bro gong them in. Hope it works out for you