Here in South Africa the biggest growth for WTS is coming from "migrants" from the countries to the north of us who have come to find the Pot-O-Gold they think exists here.
So there has been huge growth among French speaking groups eg from the DRC Congo with complete congregations being formed. Also folk from Zimbabwe [who are very well educated and articulate] and Malawi. Here too with complete native language congregations being formed. Zimbabwean and Malawian folk are highly valued in the workplace because of their positive work ethic, honesty and trustworthiness. I employ a Malawian JW housekeeper. She is a very good worker and incredible personality.
A lot of the growth within the migrant groups is due to the "promises" within the WTS message + the kinship they find with their fellow country-men. [Association/Security/Networking]
Once a migrant finds accommodation this also provides the nucleus for the rest of his family/friends to migrate as they have a place to stay. Often there are several families staying in one house/dwelling. They look after each other - which is quite understandable and if one is a JW he will of course introduce them to the inner sanctum of the congregation. Of course they also receive assistance from the rest of the congregation too. So the power of this kinship is a big drawcard and has also contributed to the +growth in JW numbers down here.
Many migrants are here on a Refugee/Asylum status or are given Temporary Work Permits. Some like the Zimbaweans apply for study permits where possible. The congregations do not make a fuss about ones residency status but you would not be appointed as MS/E if your paper work is not "legal".
I personally think WTS thrives on the migration of people all over the planet.
Religion is a helluva business model. [Old Jewish business maxim: "Never use your own Money!"]