There are a ton of countries which pretty much accept anyone who wants to migrate there. Virtually all people who move to a new country tend to be rich. A rural farmer in china's north west doesn't know how to move to Canada.
There's a brain value and potential for population decline in places. Germany for example is going to have a terrible 2030s. They pretty much will let anyone in because they simply need brains. They are tremendously wealthy and can pay for integration costs.
UAE is another, they are oil rich and can afford to bring anyone in. They will invest in anything. They have what 50 trade zones? This basically just completed: https://en.wikipedia.org/wiki/Dubai_Maritime_City
Very smart of them to do this as well. there's a finite amount of oil, but brain power is infinite money.
>having the culture of tribe "b",
You never explained what this is. It's possible you have a culture that is hostile to immigration?
For example socialism is quite hostile. USSR didn't trust border guards. That was the job of KGB to make sure nobody is crossing their borders. The only people who ever got into the USSR were top brains. Much of their actions were quite related to orchestrating brain drain. They also made big effort into education. It's difficult to offer social programs or welfare net and also be bringing in the unwashed as it were.
The thing as well. Even in countries where immigration isn't open. If you talk to an embassy and make the case for how you might contribute to their country. They will just let you in.