How it works
By default, Fillout emails the form creator when a meeting is booked, and Google Calendar sends the respondent a meeting invite with details.1
Create a custom emails
If you’d like to also personally email your respondent, you can add a custom followup email in your form Settings from the top menu. Click 
Notifications
in the left side followed by Custom emails.
2
Edit your message
Choose the 
Thank you email
workflow and Edit
your message. More details here.