In Close Encounters of The Third Kind, what happens with the people that leave the earth with the spaceship in the ending? Are they staying there with the extraterrestrials and living happily?
To date, Close Encounters of the Third Kind is a stand alone movie, not canonically connected to other stories. Its hard to find a definitive statement that there is no canonical material connected to Close Encounters, but I've based my statement on some brief research:
To summarize, it appears that Spielberg decided that he did not want to make a sequel to Close Encounters, but also did not wish the studio to make one without him either. So it would appear partly to prevent the studio going to someone else, he did some development work on a movie initially called Watch the Skies (the same name given to Close Encounters at early stages), later dubbed Night Skies. This was given a more horror treatment than Close Encounters. Material from this work went on to inspire E.T. and Poltergeist. However it is not clear from anything I've read that this was intended as a direct sequel to the Close Encounters story.
For further reading the wikipedia entries on Close Encounters and Night Skies are somewhat backed up by articles written by the author of The Making of Close Encounters.
Close Encounters ends with the selected humans voluntarily joining the aliens on their ship because they believe they are friendly, and are not shown as malevolent in any of the scenes of the movie. It is implied in the movie that the aliens are currently friendly.
That said, because there is no other 'in universe' material to work with, we can have no idea whether their intentions are good or not. The question is not categorically answerable.
The humans are alive and kicking, provided you read german. Rainer M. Schröder authored three young adult novels (of rather dubious canonicity I must hasten to add) that are recognizably sequels to Close Encounters (same ship, same aliens, same backstory). They were published in the early eigthies by Schneider Bücher (now an imprint of Ehapa) and chronicled the human's adventures on distant planets (plus the atmosphere in the alien spaceship bestowed immortality on the earthlings, so we can be glad we got away with three books). I'm not even sure Schneider Bücher did buy the rights to the story, they might have changed just enough to avoid copyright issues (alas I cannot recall to much detail of something I've read 30 years ago).
I partially object to the premise of the question. To my recollection, although a selection of volunteers is presented for the aliens to take aboard, only Richard Dreyfus's character is actually chosen and brought aboard. It is plain that the doors close immediately afterwards. As to what happens to him, one can only speculate, although I doubt if the aliens did him any harm.