I have always read to introduce new chickens at night, after everyone has gone to roost.
According to the literature, when the flock wakes up (so to speak) the established chickens will see the new birds (intruders) as more a part of the flock instead of outright intruders.
Chickens it seems have quite a strong pecking order and don't like change.
Whenever I have introduced new birds during daytime, the same things happens. When I have introduced at night, it isn't perfect, but does seem to go better.
What I have found to work here is I will pen the new chicks/birds next to the established flock for a few weeks. Gives the new chicks a chance to acclimate without getting picked on as well as overcome the "scary outdoors" fear too. And it lets the established flock to get used to the youngsters/new bird without feeling too threatened.
Even so, when it is time to put them all together, I still do it at night.