In your Select you need to switch to Text mode, so it returns a simple array of emails (not an array of objects with Email property).
Then, when you use union(body('Select'),body('Select')) it will result in the following output.
[
"person1@email.com",
"person2@email.com",
"person3@email.com"
]
Finally, you can use the following expression to join the emails together, separated by a semicolon.
join(YOUR_ARRAY_OF_EMAILS, ';')
//Result
"person1@email.com;person2@email.com;person3@email.com"
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