script

 // This was written for Google Apps Script so the emails are sent through the account that created the Google Sheet. 
 //To use elsewhere you will have to modify the mail function near the end.
 
 // getting data from our Google Sheet
var ss = SpreadsheetApp.openByUrl("GOOGLE SHEET URL").getSheetByName("SHEET NAME");


    // Create variables with the inforamton for the email message. HTML must be seperate from text for some reason
    var subject = '';
    var message = '';

var values = ss.getRange('B1:B').getValues(); //This code gets all member emails in column B from row 1 and below
var students = [].concat.apply([], values).filter(String); //This gets only rows that has emails and not the empty ones.
var numOfGroups = Math.round(students.length/25); // how many groups of 25 emails we will send based on total number of members

var run = 1  // sets the number of runs and gets updated each time through the loop
var startRow = 1;  // set the row of emails to start with and this gets +25 in each run of the loop


while (run <= numOfGroups){    // while the run number is below the total number of groups to be created
let groupOfNames = [];  // create an empty arroy to store 25 emails each time

for(x = startRow; x<= run*25; x++){    // we multiple the run number by 25 to get the end row for each group of 25
      let studentEmail = ss.getRange('B'+x).getValue();    //we get the student email each time through
      var group = groupOfNames.push(studentEmail);     // we add that student email to the array or list
      startRow++;        // we count up tp 25  and then add that to the start row for the next time through the loop
    }   // end for loop


    var group = groupOfNames.join();    // we use join function to make the array into just a string of emails

  MailApp.sendEmail('', subject, message, {htmlBody: message, bcc: group } );   // GMAIL free accounts have limited (<100) emails per day, but other email accounts may allow  you send more


    var post = ss.getRange('F'+run).setValue(group);   // optional, we post this list of 25 emails to collumn F of the spreadsheet just to see who was sent emails in each of the batches

    run++;    // we add a one to our run number and then start the while loop over again, until the run is equal to the number of groups we are creating

}  // end while