Contacts can be added to a campaign at any point in time. There are multiple ways of doing it. If you have to add a few contacts to a campaign, try Approach one. If the number of contacts are large then Approach two is recommended.
1st Approach
Step 1: Go to Audiences
Step 2: Find the contact by quickly searching for their name, if you don't have the contacts already, import them first.
Step 3: Mark the checkbox against them
Step 4: Click on the "Add to Campaign"
Step 5: From the drop down menu, Choose the campaign
2nd Approach
Step 1: Go to the Campaign that you would like contacts to be added to.
Step 2: Select on the Copy id.
Step 3: Go to Audiences and click "Import Audience"
Step 4: Add a name for the Audience, and Click on the button "Click to upload your file or to add manually"
Step 5: Add a Column called campaign ID to your excel file or use the spreadsheet in the pop-up window
Step 6: Import the contacts via the spreadsheet with the Campaign ID added against each contact. Refer to our import guide for details on how to import contacts using a spreadsheet.