Automation send settings

Have a mail drip campaign set up with automation that includes several messages. The first message is an "introduction" and the 2nd email is a follow-up. I have created a sync with CRM that the drip will be sent to, but I have already sent the introduction manually to a portion of my list. When I activate the new automation, will EVERYONE on the new list get the intro email? Or only people added to the list AFTER I activate the campaign? Please advise.