As a workaround for now, is there any reason I couldn’t share a single iCloud account to collaborate with a small group (ie my spouse and kids, using Family Share for example)?
That should work just fine indeed.
Another option is to share a Dropbox Account in Agenda. (I am pretty sure you can use a different Dropbox account outside Agenda)
Back around, any reason to not be able to share or collaborate with the new iCloud Folder Sharing??
Yes. We don’t use iCloud documents for sync at all. We use a different technology called CloudKit.
We hope to introduce sharing/collaboration at some point. It’s a big project though, so could be a while coming.