Hello,
You may need to use this solution:
https://powerusers.microsoft.com/t5/Building-Flows/How-to-convert-Array-variable-to-string-to-use-in-send-email/td-p/382120
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Flow details:
I suppose you have a list in sharepoint where people can add items there and there is a column with type Person where you can define if you allow multiple entries or not;

The result would be something like this:

Demo/test
1) Initialize variable
Trigger: when an item is created or modified (you can verify if a specific column has changed to check if someone was assigned - but this is just a detail for your flow);
Initialize variable: created a string "emails" to hold all the e-mails in a specific column called "Person" with type "Person or Group"

2) Loop with "Apply to each" for every Person's email and append the result to the variable
Apply to each: used the below expression to check the column "Person" for details; it will loop for all the People added here.
triggerOutputs()?['body/Person']
Append to a string variable: used the below expression to add to my variable "emails" all the results from my loop - only the e-mail part; Then added ";" because outlook requires ";" between emails.
items('Apply_to_each')?['Email'];

3) Sent the e-mail using the variable "emails".

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Flow overview:

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Thank you,
BR