Looks like you solved this one but here is another thought related to it.
We often build custom maps for agencies that are not public maps and frequently have issues getting outside organizational accounts successfully added to the proper Map Group. There are usually 2 settings that can be a problem. One is the Organizational Security setting that Mike referred to where you allow sharing with outside your organization (in our case we don't allow anonymous access). The other setting is in the ESRI profile for the target user, sometimes they will have that set to be not discoverable, and so it prevents you from "finding" them in the database to send the Invite to join the group.
I also heard that the Explorer version will go away in the summer update but that the functionality will be built into the standard viewer. If you used the Presentation functionality, that is replaced by the ESRI for Office application, and won't be built into the Online Viewer.