You can place people into a specific campaign by using the Request Campaign single flow step.
The campaign you are placing the person into must be active and have the Campaign is Requested trigger.
In the Database, find and select the desired person/people.
Click Person Actions, hover over Special, and select Request Campaign.
Select the campaign you want the person placed into and click Run Now.
On the upper-right of the screen, you’ll see when it’s done. Click View Results.
Your person is now in the Requested Campaign.
Use the Campaign is Requested trigger or filter to find people affected by this flow step.