First, make sure the Mailchimp integration is enabled for the campaign at hand. You can find instructions on how to do that, here.
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Then, visit your campaign Dashboard and click on "Import". There, you need to choose the "Mailchimp" icon and click on "Import".
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As a next step, you're required to choose a list to import from the drop-down menu, or select all your contacts (careful here - if you want to upload specific contacts, you need to create a list in Mailchimp first, otherwise, you'll only see the "All contacts" option).
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​Please note that we can retrieve up to 50 lists on that modal so if you cannot find the list you're looking for here, we suggest that you proceed to import these contacts using the CSV file method, as described here.
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Before initiating the import process, Viral Loops will warn you about sending the Welcome email to these imported participants (if it's supported by the template you're using). If you don't want them to receive an email, you need to disable it from the campaign notifications first and then proceed with the import.