In order to construct a string from the selected users you could call a microflow on the select action button. In this microflow you could pass a list of selected users as the input parameter. Create an empty string variable (set value as ‘’) and call it $To. Next use a loop activity and pass the list of selected users as input in the loop, change the string variable with the users email id ($To+’,’+$IteratorUser/Emailid). Now call the Send Email submicroflow and pass this string as the To or CC parameter as per your requirement.