If you do not want to charge your members for booking a guest ticket, you will need to grant them access to free Guest Ticket bookings via your membership plans.
How to do this:
Go to Settings > Membership Plans and select the membership plan(s) that grants FREE Guest Ticket bookings (if more than one membership plan offers free Guest ticket bookings then you'll need to follow these steps for each one)
Click on the Activity access tab and select the Guest Ticket activity (or activities if there is more than one).
Then click Save changes.
From then on, all members with this membership plan will be able to book a Guest Ticket for free.
On to:
Step four - Add Guest Ticket Activity to the Diary