To make your campaigns reach out to more people and run for a longer duration, you can add one or multiple lists to an active campaign. Here are the steps:

Click on the campaign name to add more lists to an existing campaign. Please check the image below:

On the campaign overview page, click "Add List" button under the campaign lists section. Please check the image below:
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Select the lists you want to add from the popup window and hit the 'Add' button. Please check the image below:
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There is an option to add one or multiple lists from the campaign #contacts page as well. Please check the image below: