Just to be clear - none of this theory is based on any hard evidence or even implication in the finale, just an idea I had to reconcile infant Aaron and Penny being in the chruch.
Regarding Aaron and Penny, because those are the two people who don't seem to fit into the mix in the church. This group is together because, as Christian said, "the time [they spent with each other on the island] was the most important time in their lives." As we know, Penny was connect to Desmond, but didn't even spend a day on the island. I mean, if Penny is there just because Desmond is there, there should likely be some other loved ones there as well.
My thought is that since we're talking about some sort of afterlife, traditional reality boundaries don't apply (like time, as we learned), and that allows for individuals to "move on" in multiple afterlife realities all at once. So Aaron moves on with Claire in this instance because of his connection to her and her connection to the group (hence why he's an infant - because that's how this group knew him on the island), but in another simultaneous afterlife reality, Aaron is a grown man moving on with his older mother Claire and other people important to him in his life.
So if you talk to the Claire ghost who's already moved on and ask her with whom she moved on, she'll have multiple memories of moving on, with different groups. We were just seeing the one group that was relevant to the story we watched. Just an idea.