Add Subscribers to Hidden Mailchimp Group via Signup Form

Working on some internal marketing R&D for us I could not find any documentation on how to accomplish the following with a Mailchimp Group:

1. Using a standard sign-up form
2. Automatically add the subscriber to a hidden Mailchimp Group
3. Requiring no intervention or option selection on the user's part

The Solution:

It actually turned out to be quite easy with a little experimentation. Here are steps to repeat my results, and you can also apply this same technique to other fields and scenarios.

It requires a couple steps. First, add your Group and the options you want the Group to contain (it can only be 1 if you want). Initially make sure the Group is not set to hidden. Go to your main default sign-up form in MailChimp under Sign Up Forms > General Forms. Verify the Group option(s) are visible and then use the Sign up form URL to visit your hosted sign-up form. Now, open the raw HTML in your browser using right-click > View Source. You need to find the INPUT element for the group / option you want. It will probably look something like this:

<input id="group_8" class="av-checkbox" name="group[13257][8]" type="checkbox" value="1" data-dojo-type="dijit/form/CheckBox" />

The name parameter is the critical thing here. Copy and paste that entire input element inside your custom form. Now, use inline CSS to hide it and HTML to hard-code it to checked. You can also remove extra stuff too. The final version in your custom form should look something like this:

<input id="group_8" style="display: none;" checked="checked" name="group[13257][8]" type="checkbox" value="1" />

This will ensure that it is not visible to the user but it will automatically add them to the group defined by the name parameter that you grabbed from the form which showed it.The final step is to go back and make sure you set that Group to Hidden to make sure it doesn't inadvertently show up on other forms.

Pretty simple!