Most of the group mail that you will likely do will be done using a group email (e-blast) utility like Constant Contact, MailChimp or Campaign Manager. Although each of these offer some free usage, you'll need to pay if you want to send out mail frequently or to a large group. You'll also need to manually (more or less) maintain the group of recipeints you are targeting with your group email.
This is where having your own in-house group email utility can be useful. These utilites are not difficult to program and can save you hundreds of dollars if you need to contact a group of your website members (say for instance your sales representatives or your store managers) on a regular basis. Usually home grown group email utilities will have fewer formating options and the messages themselves will be more informational in nature (unless of course you want to pay a lot of money to your progrrammer include these options).
Keep in mind that if you do include group email on your site, you'll also need a database of names and emails to support it. We have written numerous group email systems of varying complexity and can help you find the best solution for you.