I think this is one solution you should offer, and it should be radio buttons or drop down menu.
But you should also have hidden fields for the groups so the developer can automatically add the user to a group without the user selecting it.
For example, I want to group my list in Mailchimp based on which page the user came from on my website - an option I don't want the user to select. But if you have a hidden input option already selected it will assign the user to the group that way. This is what Mailchimp offers when you code or embed your own forms in their dashboard. Here are the instructions - if you can make your plugin work like this it would be great:
kb.mailchimp.com/lists/signup-forms/assign-subscribers-to-preselected-groups