The signup flow for Calendly is not straight forward. They do ask to verify emails which sometimes results in a drop of signed-up users. However, email ids are the core for their product and hence it is quite important for them to assure that the user has signed up with the correct email id. Unlike other SaaS products, where email id is mostly used for communication, the Calendly calendar relies on the correct email id. They have given an option to the user that instead of doing email signup, they can do a Google login which is pretty common amongst SaaS tools these days.
If you choose to signup with the Email, you have to verify your email address before you can access your account. Sometimes, such verification emails see a drop in terms of new users but you can be sure that whoever remains is a genuine user and not a fake user id. So, the choice is yours that matters the most. Also, for new email domains, sometimes these emails go to the spam folder and hence for new startups I think this step should be avoided unless you are sure that your emails are delivered to the user inbox and not to spam folders.