As far as Frozen goes, they'll specifically tell you to have your whole group together when you get into the line and not to save seats, repeatedly. So that's straight-up out of bounds (but, you can always take someone out for a bathroom break). I still see people saving seats, but it's technically not allowed and honestly I don't see that much of it.
As far of the rest of it, these topics come up periodically, and you'll find a variety of opinions and hair-splitting about the number of people, time involved, etc...
Overall, for my part, I try to give people grace. Is it annoying when a large group joins a single waiting person? For sure! That happened to me twice at the turnstiles this summer, once changing us from being about 10 people back to being more than 20 people back (minutes before the park opened, after waiting 40 minutes), and that giant group was a disorganized mess who were all entering on new tickets, needing pictures, and trying to match up their tickets to individual people... Was that kind of frustrating? Yeah. Do I think those people were intentionally trying to be jerks? Nah. I think someone said, "I'll go save us a spot in the entrance line!" Maybe that person was really excited to have "helped" their family group in that way. Is it kind of thoughtless? Yeah. But it's not breaking any actual "rules," and it's not really worth the headspace of actual anger or frustration.
Disney actually encourages people with certain disabilities to join their group later in lines. So, that's something that you never know why it's happening, and again, I try to extend grace and assume it's that type of situation rather than something nefarious.
People have different values and different ideas of what is ok. A lot of things that "offend" people aren't even on other people's radar. I prefer to assume that most people have good intentions - what do I gain by choosing to be frustrated by other people's actions?