It depends. Each guest pass isn’t considered ‘used’ until the person has registered through the link you send to them. This means that if you send one to a friend and they don’t get a chance to sign up and try the app, you can send them a new guest pass link another time.
But if it’s the case that they sign up through the link and just don’t get a chance to try the app, that pass is considered redeemed and they are no longer eligible for free Premium trials. They can, of course, still experience all the content and benefits in the free version.