The setting for allowing members of the public to make bookings is found in the Access & Visibility section of your Venue Profile.
Here you will see the title 'Who's allowed access?'. The first option is the one you need. This is the option which is normally selected by default.
With this option enabled, visitors to your Skedda URL can view your scheduler without having to login. These visitors can also make new bookings, creating a user account and adding themselves to your users list in the process.
For venues with online payments enabled:
When online payments are enabled, all members of the general public will be able to visit your Skedda account URL and make bookings (as long as you haven't explicitly set your venue access to "private" in the same screen above).