Hi @Jsharkey ,
Could you please share a bit more about the Collection data you used store the visitors info?
How do you submit your data back to your SP List? Using SubmitForm function?
I assume that you use SubmitForm function to submit your form data (SharePointForm1) back to your SP List, I have made a test on my side, please consider take a try with the following workaround:

On your side, please go to your Edit form (SharePointForm1), set the Default property of the Visitors Text Input Box to following:
If(
SharePointForm1.Mode = FormMode.New,
Concat(VisitorCollection, Name & Char(13)), // VisitorCollection represents your Collection table
PlainText(ThisItem.Visitors)
)
Note: The VisitorCollection represents your Collection table, the Name represents the column in your collection which stores the Visitor name. Please replace it with actual collection name and column name within your app.
Set the Mode property of the Visitors Text Input Box to following:
TextMode.MultiLine
When your Edit form is in New Mode, the Visitors field Text Box would be populated with concatenated visitor name from your collection table automatically.
Best regards,