getGithubEmail.js

function getGitHubEmail() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getActiveRange();
  var username = range.getValue();
  var adjacentCell = range.offset(0, 1); // adjust this if the email is in a different column
  var email = "";
  
  if (username.indexOf("github.com") >= 0) {
    username = username.replace(/^.*com[/]([^/]*).*$/,'$1');
  }
  
  var apiUrl = "https://api.github.com/users/" + username + "/events/public";
  
  var response = UrlFetchApp.fetch(apiUrl);
  var data = JSON.parse(response.getContentText());
  
  var commits = [];
  
  data.forEach(function(event) {
    if (event.type === "PushEvent") {
      commits.push.apply(commits, event.payload.commits);
    }
  });
  
  var emails = [];
  
  commits.forEach(function(commit) {
    if (commit.author && commit.author.email) {
      emails.push(commit.author.email);
    }
  });
  
  if (emails.length > 0) {
    email = emails[0]; // change this if you want to use a different email from the list
  }
  
  adjacentCell.setValue(email);
}