I will chime in here with my experience, and my take.
Unless there is some reason why your travel partners might not be left alone there, (not, mature, normal,l adults) I do not see any real obligation by Princess to let them board. Easy enough for Elite to board, take care of any early boarding stuff/reservations, etc... and simply meet their travel partners somewhere as planned.
In our case, we are platinum, so we were able to board earlier. But, we were traveling with our son, who was traveling solo... Cruising with us without a friend/girlfriend. He had turned 18, but there is NO way that we were leaving him alone there.
We checked in at the counter, as he checked in with the rep next to us. Our customer service rep, like some others we have seen, was very rude, and should not be working there in that position, at all.
She noticed that our son was there. She told us to wait a second, disappeared, came back and handed us our boarding information. We didn't realize it, but because of our son, she went and changed it to regular, non-priority boarding!!! She did that on purpose, because she knew that we would boarding with our sone along with us. YES, THIS HAPPENED.
When we realized this, and they announced that passengers with the early boarding pass could go forward, we realized we didn't have that pass. I immediately got up and went to the first person that looked less hostile, and showed them that we were indeed platinum/elite. Luckily she waved us thru, with our son right with us. This meant that we could get reservation spots that we needed, etc... that we would not have gotten without our priority boarding.
Bottom line, with things like this, it will depend on the situation and who you end up encountering at check-in and boarding.