When you generate a new lead you can choose to send the subscriber's information directly to your Mailerlite account.
Step 1 - Add A New Provider
Visit the email integrations tab of your account settings, click the 'add a new provider' button:
Step 2 - Connect To Mailerlite
Select 'Mailerlite' from the Providers dropdown and enter your Mailerlite API Key as shown below.
Step 3a: Lead Capture - Select Your Group
If you are creating a Lead Capture Form then you can select which Mailerlite Group each subscriber should be added to:
Step 3b: Resource Library - Select A Group
If you are creating a Resource Library then you can select which Mailerlite Group each subscriber should be added to via the 'Optin Settings' screen: