A user (with e-mail) fills in its preferences to volunteer on one/more committees and submits the form. The event triggers userSubmitsForm which puts the user's e-mail in the appropriate column with all the other volunteers for that committee.

Volunteer

var sheet = SpreadsheetApp.getActive().getSheetByName('Overview');


function userSubmitsForm(e) {
  var user = {name: e.namedValues['Full Name'][0], email: e.namedValues['E-mail'][0],
              committees : e.namedValues['Which PDA Committee are you interested in to volunteer?'][0]
             };

  var committees = user.committees.split(', ');//space is important!
  for (var i = 0; i < committees.length; i++){
    var committee = committees[i];
    add_email(committee, user.email);
  }
}


/**
Append argument email 
in the column with first cell value equal to argument committee, 
in the first cell of this column that is empty in this row.
**/
function add_email(committee, email){
  
  //get column number for this committee
  var col_number = -1;
  var headers = sheet.getRange('A1:H1');
  for (var i = 1; i <= headers.getNumColumns(); i++){ 
    var header = headers.getCell(1,i).getValue();
    if (header == committee){
      col_number = i;
      break;
    }
  }
  
  var cell = sheet.getRange('a1');
  //append email to end of row:
  var row_values = sheet.getRange(1, col_number, sheet.getMaxRows(), 1).getValues();
  var last_row = row_values.filter(String).length;
  cell.offset(last_row, col_number-1).setValue(email);    
  
  // Make sure the cell is updated right away in case the script is interrupted
  SpreadsheetApp.flush();
  
}