You can trigger an automation based on a contact being added to an event as a guest
The automation criteria can be based on
the amount of time before the event’s start date
the amount of time after the event’s start date
the amount of time after the event’s end date
For example
E.g. if you want to send a reminder 7 days before the event,
Automation Criteria
You would choose
The amount of time before the event is
And enter the value of 7 in the This field
Automation Action
You would choose Messaging and then select SMS and/or Email and then add the message