The currently accepted answer depends on automated passport control (APC), but not all airports have APC.
I have traveled to the US with non-US-citizen companions on many occasions, beginning around 15 years ago. I've never used APC.
CBP officers have told me on several occasions to bring my non-US companions with me in the US citizens' line. More recently, I have used the diplomats' line because I was traveling with someone who was entitled to use that line.
From these experiences, I infer that the basic principle is for people traveling together to approach passport control together, and to pick whichever line they think is best as long as at least one person in the group is eligible for that line.
This obviously might not apply to APC, since APC can't process all travelers. Since I have never used APC, however, I am not in a position to offer any useful speculation about that.