Adding a step to an Automation only applies to future contacts on that Automation, except for a couple circumstances:
Contacts currently on a delay step will hit new steps if the new step is placed after the delay
Contacts currently waiting on a goal step will hit new steps if the new step is placed after the goal
Once a contact has completed an Automation, new steps will not apply to that contact.
Adding An Additional Step To An Active Automation: Delay Sequence
Step 1: Go to Automations to select an active automation.
Step 2: If you have a Delay Sequence already set up, contacts will be able to see the new Automation you will add after that delay is complete. Click the [+] icon to add another Automation Step.