Probably the official answer is that each adult should use the lane appropriate to her circumstances.
That said, in practicality, in the US and elsewhere, a small group of two or three may stay together and use the fastest line. By a small group, I mean perhaps an adult citizen with his alien parent, someone travelling with a disabled person using a fast lane, or a married couple (where one spouse is not a citizen). I am not talking about friends or colleagues travelling together.
The immigration inspectors are not specialized for any purpose, an immigration inspector in the "Aliens" lane can process a citizen and an inspector in the "Citizens" lane can process a tourist, if they so choose, and they will understand that a family will want stay together.
But this is bounded by common sense. If someone has a complicated visa process that will take some minutes to arrange, then he should probably not be in the citizens' line holding everyone up. If one traveller entering the US has a valid Automatic Passport Control slip and the other needs to be fully processed, then they will be separated.