[SalesForce] Custom button javascript error on Unexpected token ILLEGAL

Assuming you have this button on a related list/list view, you can use the {!GETRECORDIDS} javascript function to get the list/array of id of the selected records. You can then send this array to your apex class and then query the email field to get the email. YOu can do something like this:

{!REQUIRESCRIPT("/soap/ajax/28.0/connection.js")}
{!REQUIRESCRIPT("/soap/ajax/28.0/apex.js")}
var recordIdArray = {!GETRECORDIDS($ObjectType.Contact)};
console.log('the record Ids are' + recordIdArray);
var retStr = sforce.apex.execute("outbound_Emails", "SendEmailNotification",{conId: recordIdArray});

global class outbound_Emails{
        webservice static void SendEmailNotification (list<id> conId){
            list<contact> conList = [SELECT name,email FROM contact WHERE id IN : conId];
            system.debug('the selected contacts are  ' + conList);
            //you can iterate over this list to get the email and send the mail
        }
}

First off, interesting use of REQUIRESCRIPT and base64 encoding! Here's a couple of things to note:

Anything inside bang-brackets is evaluated at page load time, everything else is only evaluated once your button is clicked. Here's some code I cooked up to disable a button named "test button" if the complaint field is set to true on my case.

{!IF(Case.Complaint__c = false, REQUIRESCRIPT("data:application/javascript;base64,Y29uc29sZS5sb2coJ3N1Y2Nlc3NmdWwgbG9hZCcpOw0KdmFyIGVsbXMgPSBkb2N1bWVudC5nZXRFbGVtZW50c0J5TmFtZSgndGVzdGJ1dHRvbicpOw0KZWxtc1swXS5kaXNhYmxlZCA9IHRydWU7DQplbG1zWzBdLmNsYXNzTmFtZSA9ICdidG5EaXNhYmxlZCc7"),"")}

Just make sure you get your full statement for setting the color of your button inside of bang-brackets and it'll run at load time.