How to add group/interest for CF7 with MC4WP

You can use this PHP code snippet to set a tag:
https://github.com/ibericode/mailchimp-for-wordpress/blob/main/sample-code-snippets/integrations/contact-form-7/different-tag.php
Would that work for your situation?

Thank you. I was trying to set this up for groups/interests, in addition to tags. Is there sample code for doing groups/interests?

You can look at this example:
https://github.com/ibericode/mailchimp-for-wordpress/blob/main/sample-code-snippets/integrations/woocommerce/add-to-grouping.php

But note that is for WooCommerce, so you have to change the integration tag in the code from mc4wp_integration_woocommerce_subscriber_data to mc4wp_integration_contact-form-7_subscriber_data

Hope that helps. If you have any questions, please let me know!

Great, thank you. Can interests/groups can be changed by adding code to the CF7 form code? Or does it always have to be done using PHP?

It has to be done using php.