To invite attendees that booked in a concluded or current activity to a future activity:
Open the relevant activity and head to the All Attendees tab
Select the relevant attendees you wish to invite
Choose +More
Select Invite to Book Activity
Choose between send one invite per family or one invite per attendee:
Select which activity to invite the attendees to
Once you have selected options, click Continue. This will take you to the Invite to Book email for you to send