New Codex documentation for BP_Group_Extension

I’ve just completed a draft of the new codex page for BP_Group_Extension, which has been rewritten for the upcoming BP 1.8 release. Here’s where it’s currently living: After 1.8 is released, will be replaced with the new page, and the old content moved to its new home

If you’ve worked with BP_Group_Extension before, please dive into this new documentation to make sure it makes sense from a dev’s point of view.