While setting up a new campaign, you need to add people to the campaign either by hitting 'Add People' button or via the red "+" button if there is at least 1 contact in the campaign already:
Once you have pressed "Add People", you'll see a pop-up with two options:
Select one of these and add people to your campaign.
Note: when you use the Import from CSV option, you can enable the Skip updating existing contacts slider. In this case, the system ignores the existing contacts and does not override their data.
When you add people from existing, the prospects listed are all available to be added to the campaign, which means that any contacts associated with a different campaign will not appear. Select the desired people and add them to the campaign:
For more information about step delays, see the following article.
If you need to add new contacts to an existing campaign, simply go to your Campaign -> People tab and use the red button to add more people: