Well that answers part of my earlier questions. Notes are one way; you’ll grab Calendar’s Notes to new Agenda notes (but not keep them in sync).
So what happens if we add an invitee after? Answer: The link does show up, as a line of (confusing;-) text (at least in web based iCloud Calendar). But maybe the use will ask what that is all about, and we’ll sell them on Agenda. Wow, so much to consider when integrating; I feel bad for asking and assuming it would be easy. I can see why working in the cloud only is so appealing, as the link could be cloud (rather than local) based.
Unless you add an invitee after you’ve made the Agenda connection, in which case it may be relevant but Agenda inserted the link before you added the invitee, so, tag, you’re it.
Indeed, not easy to implement, and certainly not in a way that everyone intuitively makes sense of. Thanks for sticking with it; over time it will make better sense I’m sure.