Certain Groups filling Economic niches

What I mean by this post is what groups were most well known for filling a certain role in a nation/empire. For example Armenian traders in Safavid Persia, or Ottoman Greeks and Armenians. Can anyone else think of any groups that specialized a certain role in the society they lived in?
 
Lithuania used to have plenty.

Lipka Tatars served as city guards and reliable soldiers, while Karaites took part in usury and some in military service, for example.
 
The examples you and @Minchandre give are what sociologists call middleman minorities. Wikipedia's list there is good, though I have my quibbles (I certainly wouldn't call Azeri Turks in Safavid Iran "middleman minorities," they were more of a professional militarized ethnicity).

The Romani (Gypsies) and their analogues, like the Nomads of India, are another type of specializing minorities called "peripatetic service nomads," nomadic populations that carry out minor services, most commonly tool repair and entertainment, that their host sedentary populations require. This is how "Tinker" became a word for Irish Travelers and Romani ("tinker" used to mean "tinsmith").

Some ethnicities exploit different ecological zones that their neighbors don't care for and thereby provide important services to the majority population. These include the Hakkas of southern China, the "Sea Gypsies" of Southeast Asia, and the forest peoples of Central Africa.

Some ethnicities enter special contracts with the state because of their (real or perceived) military qualities and serve as "martial races," like the Tatars mentioned by @Augenis. The British Raj is famous for this, but most empires have employed some variety of this. The use of southwest hill tribes as scouts by the Ming empire, or the importance of northwestern Turks in the sixteenth-century Safavid empire, are examples of this.

In some places, minority ethnicities play important religious roles because their ethnic group is so closely associated with sacred authority. The best example I can think of is the role of Tibetans in Early Modern Mongolia, but Hadhrami Arabs played a similar role in eighteenth-century Indonesia.
 
1. Germanic peoples in the late Western Roman Empire were pretty militarized, I’m sure there’s other examples of a “warrior caste” in premodern societies

2. On a related note, Greek slaves in the early Roman Empire often acted as tutors or teachers for patrician children in Italy

3. European Jews during the medieval period working in finance and lending

4. Pretty sure the Mongol empire had some ethnic-based occupational niches for all the specialists they imported from the Middle East/Central Asia into China, but I could be wrong. And I guess the mogols themselves fulfilled a specific warrior caste rather than being traditional administrators
 
The Jews were already mentioned, although I'd add the Florentine and Venetians in the XIII - XIV century for banking.

Plenty of monarchies employed regiments of foreign soldiers as personal elite guards, distinct from mercenaries. Supposedly they had no links with anyone but the ruler in the country, an thus would not be involved in local politics.

-Galatian Celts were employed by the Ptolemies and the Seleucids
-Varangian Guards of the Byzantine Emperor
-Scottish and Irish Regiments of the French Kings
-Various Cossack nations in Russia
-The Papal Swiss Guards

I would exclude later colonial regiments (Gurkha, etc) because they were levied by their colonial masters.

Then after, you have migrant groups bringing certain specialty skills from their home country, i.e German migrants bringing beer brewing or French bringing wine-making, but I think that's out of scope.
 
Jews, of course.
3. European Jews during the medieval period working in finance and lending

While Jews tended right from early medieval times to have a strong presence in economical and commercial exchanges, this position began to be really disputed to the point many Jews were front persons rather than real owners and loaners for Christians. By the XIIIth it became even less relevent, as the large use of Cahorsin or Lombard for loaners and financers does point (and these weren't, of course, systematically from Cahors or Lombardy), especially giving the pervasivity of lending from the XIIth century onward (cf. La naissance du capitalisme u Moyen-Age, by Jacques Heers)

1. Germanic peoples in the late Western Roman Empire were pretty militarized, I’m sure there’s other examples of a “warrior caste” in premodern societies
I don't think we can talk about a warrior caste there, especially giving getting in and out of Barbarian ensemble was really easy to do for both Romans and Barbarians, and from the early Vth, Barbarians integrated more and more non-fighting groups. It doesn't mean that fighters didn't accounted for a large part of their population (roughly, between 1/3 to 1/2, which is definitely making them specialized groups), but not as a caste.

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Some kind of economic activities tend to be overlooked, because in a capitalist outlook, they weren't really as prestigious to be put a light on : I'm thinking especially to butchers that tends from time to time, to be made of distincts groups.
For instance, butchers in southern France in the XIIth century were sometimes assumed to have a distinct Cathar proportion : probably less because they organised themselves as such, than butchers being already more or less formally organized allowed some networking.
More recently, in fact contemporarily, the growing importance of halal butcher's shops in France in cities (with sometimes a 3/1 ratio), born out of the necessity to be provided with hallal meat and replacing former butcheries out of rivality of supermarkets (at least partially).
 
Well didn't expect to get that much feedback. Another good one is perhaps the Turks during the late Ummayad-early Abbasid. These Turks were the forerunners for the Ghazis (soldiers of Islam), while at the same time being traders into other parts of the Islamic World.
 
The Norman nobillity in England.

The European nobillity in general, perhaps nobillity worldwide in general.

Executioners in France.

Sami people
 
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