As long as the immigrants pay for what they use, they do not make the rest of the society poorer. If increased population makes the country more crowded, it does so only because the immigrants produce wealth which is worth more to the owners of land than the land is worth, and the immigrants are able to use that wealth to buy the land. The same applies to whatever the immigrants get on the free market; in order to appropriate existing resources for their own uses, the immigrants must buy them with new goods of at least equal value.
— David Friedman, The Machinery of Freedom (New Rochelle: Arlington House, 1973), pp. 95–96.
It’s hard to put it any better. Many people, although probably less than in 1973, are concerned that a large immigrant population will require the pie to be cut into smaller pieces. The fact is, however, that in order to have a piece of the pie the migrant first has to produce a piece of his own that he can use to exchange. As such, if migrants are to find living in a foreign land successful they’ll have to add to national income, otherwise they can’t join in on the network of exchanges that makes up our economy.