I'm testing setting up a SalesForce that will be using the "Group" edition and I want to make sure I'm only using what I will have available. How do I change the edition to group in a developer account?
Editions can't be changed (at least not by us). You will make your development on a developer org (which is quite close to an Enterprise Edition) and you can sign up for it here for free:
Later you will deploy your results into your group edition, e.g. as a package. You have to know and respect the limits of the group edition and if you use stuff which is not supported by GE, you will either not be able to install the package or get runtime errors later.
If you need a kind of test-environment where all the limits of group edition apply, you can do it either in a Trial or in a Test-Org which you can create in the partner portal, or in the environment hub (if you are a Salesforce Partner).
However in the Group Edition all the "good stuff" is not available, so there is not much to develop beside a few custom objects and custom fields...