Here's my experience:
I had two chicks, Tom and Jerry. Tom was killed by a raccoon and Jerry was devastated. He'd lay under the dining room table and whimper.
So I got him a couple of buddies. Slifer and Obelisk. I didn't quarantine, didn't do any of the things that you should do...but I was lucky. He loved his friends, especially Obelisk who is the same breed. They'd go flying over the fence together to eat the neighbour's grass seed, while Slifer just looked at them through the fence.
I had to rehome Jerry when he started to crow. I was the most broken up by it, but the two girls did look around for him for a while.
Last summer, Slifer died. Obelisk looked for her friend even though they weren't close for a week. She'd sit in my lap and bikbikbik and want to be patted. She was happy to be a lone queen hen.
In December Penny came home. Obelisk was NOT thrilled with a buddy, needless to say. She chased and pecked her unmercifully. Finally, we got things settled down and then Penny went broody. Back to square one...
If Obelisk gets too rough, Penny goes into her cat carrier. They have separate dishes too which helps a little.
Alot depends on the temperment of the bird.
Even if you can snag a cat carrier off of Freecycle, it would allow the newbies to see and smell the established group.