This is a great question. There are 3 states of campaign membership:
Contact has not been ever member of the campaign ever. If you place this person in the campaign, he wil go through the flow from step 1.
This person is a member right now, going through the steps. Either is between steps waiting, or between an update and trigger command (read about cronjobs). In other words: member of a campaign. When the last step of the campaign is done for this contact he will leave the campaign.
Left the campaign
This contact already went through OR left the campaign for some reason. For example disqualified itself to be a member of a segment, which was the source of the campaign.
If you set your campaign in a way that contacts can go through repeatedly, than this person is in ‘Never stage’ for this particualr campaign.
So if people are in fact a member of your campaign (not finished), and you add emails, than after a campaign:update command they will get the new campaign step, and it will be triggered to them by a campaign:trigger.
If they left, then you can can create with the following filters:
segment membership same as your campaign source
exclude membership of your campaign
If the source of the campaign is a form, than let me know, you might need to tweak some things.
You can also deep dive into the mysterious secrets of campaign triggers: