how to insert users into particular group using csom javascript

Straight from MSDN (https://msdn.microsoft.com/en-us/library/office/hh185012(v=office.14).aspx)

var siteUrl = '/sites/MySiteCollection ';

function addUserToSharePointGroup() {
    var clientContext = new SP.ClientContext(siteUrl);
    var collGroup = clientContext.get_web().get_siteGroups();
    var oGroup = collGroup.getById(7);
    var userCreationInfo = new SP.UserCreationInformation();
    userCreationInfo.set_email('[email protected]');
    userCreationInfo.set_loginName('DOMAIN\alias');
    userCreationInfo.set_title('John');
    this.oUser = oGroup.get_users().add(userCreationInfo);
    clientContext.load(oUser);
    clientContext.executeQueryAsync(Function.createDelegate(this,this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}
function onQuerySucceeded() {       
    alert(this.oUser.get_title() + " added.");
}
function onQueryFailed(sender, args) {
    alert('Request failed. ' + args.get_message() + '\n'+args.get_stackTrace());
}

Even though Marius's answer is correct, I find this a bit more clean. It takes an array of user names and a group name. Works for any number of users.

function addUsersToGroup (usernames, groupName) {
    //Get the web
    var clientContext = SP.ClientContext.get_current();
    var web = clientContext.get_web();

    //Get the group from the web
    var group = web.get_siteGroups().getByName(groupName);

    for (var i = usernames.length - 1; i >= 0; i--) {
        //Use 'ensureUser()' to get an SP_User object
        group.get_users().addUser(web.ensureUser(usernames[i]));
    };

    group.update();

    //Load the group to the client context and execute
    clientContext.load(group);
    clientContext.executeQueryAsync(onSuccess, onFail);
}

And call it with

var users = ["Peter Griffin", "Stewie Griffin"];
var group = "Family guy cast";

addUsersToGroup(users, group);