Hey @Angela2! Automations are not retroactive, so by default they won't apply to (or include) any subscribers who signed up to the trigger form/tag/etc. before the automation itself was created (and if a purchase event is one of the entry points, the same will also apply based on their purchase date).
That said, it is possible to 'manually' pull subscribers into an automation retroactively; we have a guide on that process here7 :)
I hope that helps!