Sending emails from the People tab
The advantage to emailing at the people level is that if the contact has yet to be associated with a deal in your sales pipeline, you are able to track communication leading up to initializing and closing a deal.
Send an email to a single contact by selecting the rolodex icon under the info column and clicking on their email address. It will send the email to the work email address that is listed for the contact.
When you want to email multiple people at once, use Custom List Views to filter your deals to just the ones you wish to reach out to, check the box at the top of the column of boxes on the left to select all people that fit that filter, then click on the Send Campaign. To make things easier when selecting people to send to, filter your list view in order to prevent having to pick and choose individually.