We just completed a 1000 person event registration. Only about a third of the people who registered were added to the Acajoom mailing list. I could not find them on any list nor by themselves not on a list at all.
I assume they were members of a group registration. The plugin currently does not add each member of a group registration. We'll be adding that option very soon.
We haven't had any other reports like this... please try to determine what was different about those registrants that did not get added so we can try to track down a bug. At this point, there is no info here that we can use to track anything.