How are they stored in your variable at the moment? If it's an array of emails, then you can join each of them delimited by a semicolon.
Assuming you had a variable called emails of type Array.

You can use a Join action as shown below.

Or you can just an expression as shown below.
join(variables('emails'), ';')
Either option would give you the following output.
bob@email.com;jill@email.com;andy@email.com
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