Hi there Graham,
Thanks for posting! To accomplish the goal you have for the signup form being specific to certain groups, it would be necessary to use the embed form code rather than the MailChimp List Subscribe Form plugin. With the plugin it’s not possible to have each instance of the widget form connect to a specific group on the back end, but using the embed form code it’s possible to have hidden fields pass specific data into the list when a subscriber uses that form.
We have a guide on using embed forms to accomplish this type of goal:
Determine Webpage Signup Location: https://eepurl.com/SKbn
That guide conveys the gist of the process, but when groups are in use you’d need to customize the embed form code to include a hidden pre-selected option for the Group, rather than a text field. Either way, sends can be segmented based on the data passed into MailChimp via the hidden field in the form, but this can only be done using the embed form.
If you have any questions about that guide, the best place to seek support would be with MailChimp technical support directly. While I am a member of MailChimp support myself, my presence here in this forum is to support the MailChimp List Subscribe Form plugin specifically.
Let me know if I can offer any assistance with the plugin, and have a great day!
-Kath