Multiple Opt-Ins

I am trying to create multiple opt-ins for a single mailchimp list (like they show on this website: https://www.puzzleboxcommunications.com/offer-multiple-lead-magnets-with-just-one-mailchimp-list/). Currently, people can only receive one opt-in, otherwise they get a message that they are already subscribed. I understand I can use groups to do this (each time a person subscribes to an opt-in, they join the group for that opt-in), but I am completely lost on how to execute this on wordpress. Any help would be greatly appreciated.

Thank you.