This very much depends on the age of the bird, feathering and weather conditions. Growers are usually feathered up at about 6 weeks, and if the weather conditions are mild, and they have good housing with enough of them to keep warm at night they should be fine. Saying all that if there is a drop in temperature overnight a bit of heat, eg. an electric hen might come in useful.
It is also a good idea to "harden" off youngsters, so there isn't a major temperature or environmental change to stress them or challenge their immune systems. Some people have them outside on a fine day, then bring them in at night, then when they are hardened off, leave them out overnight, thus a gradual process of acclimatisation. When you first put growers out, they should be curious and active, watch out for signs of huddling, fluffing up, watery droppings which would indicate stress or illness in your youngsters.
Finally, it is not advisable to mix youngsters with adult birds, no matter how tempting! They are likely to get bullied or succumb to illness.