After sustaining a new RS presidency on Sunday, I tried to add names to the "Available Preparers" list. Both names are listed under "Status" as "Pending". The pop-up info window only says that a preparer must have an LDS Account. I know they both do, as LDS Accounts were necessary for their previous callings--at which they both excelled.
Maybe "Pending" means that some process will eventuate their access. I'll wait a day or two to see what happens.