You will see the list of groups (communities/enterprises) that you are a member. The groups that you can manage are listed with a link “Manage”. Assuming that you are a community administrator, click on “Manage” link.
In community control panel, click on “Manage members”
You will see the members list of community and the send e-mail button. Click on the button.
Fill in the “Subject” field with the subject of the e-mail and the “Body” field with the content of the e-mail you want to send. Then, click on “Send”