Do my invitees have to use Scedule?

No. If you also want to consider the availability and preferences of non-users, then simply start a Smart invitation by selecting 2-6 dates instead of just one. All participants will then receive an invitation (non-users via e-mail, Scedule users in addition via push notification) to an interactive poll. They can decide upon their preferred dates. Before you as the organizer can decide on a final date.

In the case of a direct meeting invitation, each participant will be invited via your standard calendar. Nevertheless, the availability of Scedule users will, of course, be taken into account as usual.

And by the way, you can invite non-users to download Scedule for free so that their availability is automatically being considered.