If you would like to create a class which is free & included with all membership types, simply head to Manage > Classes > click the blue 'Add' button in the far right corner. Fill in the class information as normal (name, description, trainer, etc.), and under the 'Price / Eligibility' section, select 'Single Price' and set the price for 'Drop In Clients' and 'Members' to '0', as per the screenshot below.
It is also advisable to utilise the start & end date options when setting up this class, if it is a one day event which does not recur the following week.
However, it is extremely important to note that, if any of your memberships are credit-based, booking a class will always deduct a credit, where applicable.
If this is the case, and you would prefer not to deduct a credit, simply apply the steps above to set up your free class as a course instead. This will not deduct a credit; just charge the price which is set.