@Billy Walton I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer.
Issue:
You observed that under the Developer tier. All of the documentation you can find states that when you edit a "Product", you can click the "Access Control" tab and there should be a list of toolbar buttons one of which is "Add Group". You have logged in as the owner of this APIM instance and this isn't showing. So, you can't add the "developers" group to the product.
Solution:
You found that "Owner" role was inherited from you being the owner of the subscription. You added yourself to be the "Owner" of the APIM instance and that solved the problem.
Thank you again for your time and patience throughout this issue.