Here is how you can merge audiences using Mailchimp. Basically, login to your Mailchimp account. And go to the Audience. And go to the Audience Dashboard. In here, you will find Manage Audience. Click on View Audiences. Here, you will find all your audiences. Now, click on Manage Audiences. Scroll down, and you will find Combine Audiences at the bottom. Choose the Audience you want to merge. And Next. And type Confirm to combine audiences.

Please note, you cannot combine audiences if you are sending some newsletter. In that case, try after 7 days, and it will work all fine. Thank you very much.