In 2017 (Port Canaveral) our friends were we in a separate cabin and were not Platinum but they went with us to the Platinum check-in and were able to board with us. I didn't just assume they could, as I asked the CM if they could check in with us or did they have to check in at the CC desk. She said no problem and we were good to go. If I had to guess I think it is up to the CM and also I would think it would depend on how many you take with you to the desk.
It's not fair to put the desk agents in the position of having to make an exception or risk getting yelled at/complained about when they (rightly) say no by asking them for special treatment.
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