Form without fixed number of input fields

I'm working on a form that allows a user to invite new users to my service. So basically a user should input email addresses and my website will send invitation emails. Now when user visits that form they can see an input element and button that when clicked will add another input element. Something like this (please assume that user already clicked the button and they have two input fields):

Is it a good solution for a form without fixed amount of possible inputs? I can see at least two more options:

1. show user more input fields at the beggining
2. dynamically add next input field once user starts writing in the last one

In 2. solution I'm afraid that that behaviour can be unexpected and confusing for users.

In first solution I wonder if users will think that they have to provide 5 addresses whereas they should be able to submit form with only one valid address...