It’s very unlikely that this will change, because it’s the base of how diaspora works. You decide on which people you share something at the moment when you share something, and it’s sent to these people. You can’t remove somebody from this list after you shared it and you can’t add somebody (well, in theory you could, but I come back later to that). Diaspora isn’t a wiki where you can manage permissions with groups and give a group permissions to view a section and then just add somebody to a group. It’s also not something like nextcloud/dropbox/whatever where you can create an album with photos and then send somebody the link and then can just send the same link to more people. When you need something like this, then diaspora probably isn’t the right tool for you.
You need to add your family first and then you can share the photos with them. When they don’t want to register then they don’t see the photos. You can see it like in real life when you tell them to come over to watch the photos, and when they don’t come, then they don’t see the photos. When you show more photos the next day and they come over this time, then they see the new photos, but not the photos from yesterday.
Now back to sending an existing post to new people. In theory this is possibly (with the protocol), but sending all your past posts to new contacts would add a lot of load to the network and to smaller pods, because it’s not built for that case. And maybe not everybody is happy with sharing old stuff with new people (when you have a photo with some people, and they agreed that you share it with a group of people, they maybe don’t agree with sharing that old image with new members of the group).
Because of these reasons/problems it’s very unlikely that this will change.