Добро пожаловать! (Dobro pozhalovat!)
This is a formal greeting, similar to "Welcome!" in English.
Here are some other ways to say welcome in Russian:
* Рад(а) вас видеть! (Rad(a) vas videt!) - "Glad to see you!" (informal)
* Приятно познакомиться! (Priyatno poznakomit'sya!) - "Nice to meet you!" (informal)
* С возвращением! (S vozvrashcheniem!) - "Welcome back!"
The best way to choose the right greeting depends on the context and your relationship with the person you are welcoming.