If this is ever of use to anyone else I have solved my issue with help from Stack Overflow; updated script is below:

function updateManyUsers() {

varsheet = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Sheet1');

var [headers, ...rows] = sheet.getDataRange().getDisplayValues();

Logger.log([headers,rows]);

varusers = rows.map(r => {

vartemp = {};

headers.forEach((h, j) => {

if (r[j] != "") temp[h] = r[j];

});

returntemp;

});

varuser = 'morris.coyle@retacted/token';

varpwd = 'redacted';

varoptions = {

'method': 'PUT',

'headers': {

'Authorization': "Basic " + Utilities.base64Encode(user + ':' + pwd)

},

'payload': JSON.stringify({ users }),

'contentType': 'application/json',

'muteHttpExceptions': true

};

varresponse = UrlFetchApp.fetch(url, options);

Logger.log(response.getContentText());

}