What column type do you use in your SP list?Is it one Person type?
I test with Person type, allow multiple selections, as next screenshot shows:
Firstly you should add your O365 oulook from (View->Data Source->Data)
Add one ComboBox control, and ComboBox1.Items:Choices('20190328sendemail'.Person_M)
Add one button control, and Button1.OnSelect:Office365.SendEmail(Concat(ComboBox1.SelectedItems.Email,Email &","),"send from app","hello") ("send from app","hello" is the subject and body of the email for test, you can change on your side)