Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
I was having a similar issue, and was able to do this to refresh the invitee list and remove older invites. The other stipulation, a few “event type” changes and I had to update that event to the newer “event type” version first, for the Add Unit/Subunit to work.