THE ETHNIC DEMOGRAPHICS OF ROME
Only a small proportion of any non-Roman origin peoples living in Rome, or the Roman Empire, were citizens until all residents of the empire were made citizens automatically by an edict of the Emperor Caracalla in 212 CE. This means that most immigrants in Rome were not citizens; many were not there long term either – some settled permanently, some for a few years or even less. Those without Roman citizenship, or any of the lesser legal statuses that gave you some rights (like Latin status), were called peregrini (singular: peregrinus).  and were always vulnerable to large scale expulsions from the city at the whim of the authorities who occasionally like to do such things to protect the morality of the Romans, as they had no legal right of residence in Rome and could be pushed out at will.
Most importantly, many non-Romans in Rome were not immigrants, but enslaved citizens from various peoples. Although some may have participated in immigrant communities to some degree, as the enslaved, the freed, and the free born did join various associations together, many arrived in Rome deeply traumatized, having been captured too young sometimes to know their language or where they had been first enslaved. (They had usually been sold on and split up a number of times, as well.) Under those conditions, forming a community along ethnic or tribal lines, even if they encountered members of the same group, would be difficult, although there were exceptions.
As the centre of an empire, and a city of over a million people, Rome constantly attracted new groups and individuals. However, people also came to Rome as slaves and either remained there because they stayed enslaved until they died, or were not able or unwilling to return home after they were freed (home might well have also been pretty much destroyed too). Some celebrated this:
Rome can, in fairness, be called the nation of the world. You could not far wrong who calls the city of the Romans a microcosm of the of whole world, since here you may see every other city organized collectively and many others separately…You would more than a year to count all the cities which are to be found in that megacity of Rome, there are so many.
Athenaus, Deipnosophists 1.36
It’s hard to say what level of group identity most ethnic or immigrant groups managed to maintain in Rome. For many, it seems to have often been lost in a generation, at least going by the languages and formats of their inscriptions. The exception were those with strong group identity, and particularly the Jews. The following description of the funeral of Julius Caesar is rare in that it shows a number of different groups acting together:
The bier on the rostra was carried down into the Forum by magistrates and ex-magistrates; and while some were urging that it be burned in the temple of Jupiter on the Capitol, and others in the Hall of Pompey, on a sudden two beings with swords by their sides and brandishing a pair of darts set fire to it with blazing torches, and at once the throng of bystanders heaped upon it dry branches, the judgment seats with the benches, and whatever else could serve as an offering. 4 Then the musicians and actors tore off their robes, which they had taken from the equipment of his triumphs and put on for the occasion, rent them to bits and threw them into the flames, and the veterans of the legions the arms with which they had adorned themselves for the funeral; many of the women too, offered up the jewels which they wore and the amulets and robes of their children. 5 At the height of the public grief a throng of foreigners went about lamenting each after the fashion of his country, above all the Jews, who even flocked to the place for several successive nights.
Suetonius, Julius Caesar 84.4-5
Others, however, resented the constant influx of new groups and their customs and the dependence of Romans on non-Romans for all their basic needs.
We walk with the feet of other people, we see with the eyes of other people, trusting to the memory of others we greet one another, and it is by the aid of others that we live. The most precious objects of existence, and the chief support of life, are entirely lost to us, and we have nothing left but our pleasures [delicias] to call our own.
Pliny the Elder, Natural History 29.1.8
Bibliography and Further Reading:
This is a huge topic as you might imagine, and we can only touch on a few things in this anthology, and mention a few texts that might help you understand a little bit about the general situation.
Mathisen, Ralph. 2006. “Peregrini, Barbari, and Cives Romani: Concepts of Citizenship and the Legal Identity of Barbarians in the Later Roman Empire. The American Historical Review 111: 1011-1040
Although it concentrates on periods later than this anthology is interested in, it is very clear and gives a good discussion of issues around the citizen status of various newer groups of citizens.
Noy, David. 2000. Foreigners at Rome: Citizens and Strangers. Swansea.
Looks at various groups of foreigners in the city of Rome. A classic work; Noy also writes a lot elsewhere on the topic.
Sherwin-White, A.N. 1939; second revised edition 1973. The Roman Citizenship. Oxford
A classic, but very dry and not really that accessible unless you know a fair bit about Rome already.
- Literally 'foreignors', it was used for non citizens who lived in Roman territory. The slaves of these people did not gain Roman citizenship automatically upon freedom, even if freed in the Roman. ↵