===================== Updated Answer =====================

Please following steps:

1.Add a Trigger "When an item is created or modified"

2.Add an Action "Get item"

3.Add an Action "Data Operation -- Select"

• In 'Select' Action 'From' (input for select) will be the 'Attendees' Field.
• Map Key 'Attendees' to Value 'Attendees Email' (as only email can be used to send a mail.)

4.Add "Initialize Variable". I named it as 'OriginalAttendees'. Type as 'Array' and Value will be the Output from previous 'Select' action.

5.Add a Control "Condition". The condition is "Attendees Email" equal "null"

6.If yes.

• Add an Action "List group member" and select your group name.
• Then, add an Action "Send an email (V2)"

7.If not.

• Add 'Apply to each' where the output of variable 'OriginalAttendees' is entered.
• Then, add 'Apply to each' where the output of variable '(Get item) Attendees' is entered.
• Next, add "Append to array variable".

Name:OriginalAttendees Value: Attendees Email

• Lastly, Add an Action "Send an email (V2) "

Please following steps:

1.Create the "Person or Group" Column

2.Create MS Flow

• Add a Trigger "When an item is created"

• Add an Action "Get item"

• Add an Action "Data operations - Select"

• Add an Action "Initialize Variable". I named it as 'OriginalArrayAttendees'. Type as 'Array' and Value will be the Output from previous 'Select' action, we get only the Attendees Mail address in the array variable.

• Add an Action "Initialize Variable". I name it as 'AttendeesEmail'. Type as 'String' and Value as blank at this stage.

• Add 'Apply to each' where the output of variable 'OriginalArrayAttendees' is entered.

• Add an Action "Data operations - Compose" and in expression enter "concat(item()?['Attendees'],';')"

• Add an Action "Data operations - Compose"

• Add an Action "Set Variable"

• Add an Action "Send an email". In the "TO" field it will be the 'AttendeesEmail' variable. Rest of the body you can enter the way you want.

3.Save and Test it.

More information, please refer to:

• Microsoft Flow | Send mail to Multiple recipients from SharePoint Person Or Group field.

Please following steps:

1.Create the "Person or Group" Column

2.Create MS Flow

• Add a Trigger "When an item is created"

• Add an Action "Get item"

• Add an Action "Data operations - Select"

• Add an Action "Initialize Variable". I named it as 'OriginalArrayAttendees'. Type as 'Array' and Value will be the Output from previous 'Select' action, we get only the Attendees Mail address in the array variable.

• Add an Action "Initialize Variable". I name it as 'AttendeesEmail'. Type as 'String' and Value as blank at this stage.

• Add 'Apply to each' where the output of variable 'OriginalArrayAttendees' is entered.

• Add an Action "Data operations - Compose" and in expression enter "concat(item()?['Attendees'],';')"

• Add an Action "Data operations - Compose"

• Add an Action "Set Variable"

• Add an Action "Send an email". In the "TO" field it will be the 'AttendeesEmail' variable. Rest of the body you can enter the way you want.

3.Save and Test it.

More information, please refer to:

• Microsoft Flow | Send mail to Multiple recipients from SharePoint Person Or Group field.