In your Shopify admin panel, go to apps and then click Visit Shopify App Store. Search for MailerLite and click MailerLite Email marketing. After you get the plugin, you will be taken to the Shopify admin panel where you will need to click install app.
Click on the MailerLite integration page link below the input field where you have to enter your API key. Copy the key and paste the it in the input field. After that is done, select the existing subscriber group or crete a new one. Make sure to click Select. This will prompt you to choose whether you want to sync all of your current customers or the ones that opted in recently.
You will notice new segments had been created in the Shopify admin panel. You can also see them inside MailerLite. Just go to Subscribers, select one of your synced groups and click on Segments.
You can leave MailerLite Popups on. This will allow you to create popups using MailerLite and include in any page inside Shopify Store. No coding required.
If you want to change the group used for your Shopify store, click on the top right Manage button and select the specific group. As you see you can only have one group at a time. If you would like to change your MailerLite account associated with your store, click Manage again and select change MailerLite account. Now all you need to do is to paste the API from an another account.