Hi,
Please follow the instructions for the mass email:
https://help.salesforce.com/HTViewHelpDoc?id=email_mass.htm&language=en_US

There are some limitations:
https://success.salesforce.com/answers?id=90630000000hOXbAAM

The reason you are only being able to add 500 users is due to the following:
Outbound Email: Limits for Single and Mass Email Sent Using Apex
Using the API or Apex, you can send single emails to a maximum of 1,000 external email addresses per day based on Greenwich Mean Time (GMT). Single emails sent using the Salesforce application don't count toward this limit. There’s no limit on sending individual emails to contacts, leads, person accounts, and users in your organization directly from account, contact, lead, opportunity, case, campaign, or custom object pages.
When sending single emails, keep in mind:
You can send 100 emails per SingleEmailMessage.
If you use SingleEmailMessage to email your organization’s internal users, specifying the user’s ID in setTargetObjectId means the email doesn’t count toward the daily limit. However, specifying internal users’ email addresseses in setToAddresses means the email does count toward the limit.
You can send mass email to a maximum of 1,000 external email addresses per day per organization based on Greenwich Mean Time (GMT). The maximum number of external addresses you can include in each mass email depends on your edition:
Edition External Address Limit per Mass Email
Personal, Contact Manager, and Group Editions Mass email not available
Professional Edition 250
Enterprise Edition 500
Unlimited and Performance Edition 1,000

Reference:
http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_gov_limits.htm (at the very bottom)

Thanks,
justin~sfdc