I don't like having a calendar out there under https://needtomeet.com/myusername. Rather I would suggest that you create an option where you can share your calendar for a specified period of time. For example, I would say that I want to share my calendar with email@example.com for the next 15 days. It would create a URL with a hash that is only good for the next 15 days and I could share it with firstname.lastname@example.org. The hash might look like https://needtomeet.com/myusername/K*93jU&99124awB9 but would only work for 15 days and then it would say expired.