Either let them all out together or keep them all contained, but only letting a couple chickens out at a time then putting them back in with others that have been locked up all day will shuffle things around. There is no reason to only let a couple out at a time to get them used to things. They can all go out together just fine. People that let only a couple out at a time do that more to make themselves feel better, but it does not benefit the chickens in any way segregating them and then putting them back together on repeat.