Nope! Your friends and colleagues can respond to your scheduling links without signing up for a zcal account. That said signing up for a zcal account can make the scheduling process even faster. Registered users benefit from seeing calendar overlays on invites and polls, allowing them to see their calendar events on the scheduling page without having to switch tabs to check their calendar app.
zcal supports Internet Explorer 11. However It runs best on modern browsers such as Chrome, Firefox, Safari, and Edge.