You have an automation (a.k.a. drip campaign) finalized and you realize you need to add more contacts to it. The answer to this question depends on the campaign type of the kickoff email for the automation and if the automation has already been started.
1. If the kickoff email is a regular campaign type, and the email HAS NOT been sent yet, and the automation HAS NOT been started-
As long as you have not started the automation or sent the regular campaign that will kickoff the automation, you can add contacts to the segment or category assigned to the kickoff email campaign.
2. If the kickoff email was a regular campaign type, and the email HAS been sent, and the automation HAS started-
You cannot add contacts to an automation that already kicked off from a regular campaign type that has already been sent.
3. If the kickoff email is/was a triggered campaign type-
You can add contacts to the triggered segment anytime and they will be picked up in the automation the next time the triggered campaign runs.