Want to set up a pop up form to get Newsletter subscribers - (we a Mailchimp account)

Is there a way to set up a pop up which appears when a user arrives at our website, to get them to fill their details and subscribe to our monthly newsletter? We do have a Mailchimp account so integration would be a possibility perhaps?
Any advice is much appreciated.

Have you taken a look at this WordPress plugin?

It works well with Gravity Forms. I recommend checking out the free version first to see if you can make it work for you.