Formal:
* Bienvenidos de nuevo: This is the most literal and formal translation.
* Bienvenido de nuevo: If addressing one person.
Informal:
* ¡De regreso! (Welcome back!) This is the most common informal way to say it.
* Qué bueno que regresaste. (It's good that you came back.) This is a more friendly and welcoming way to say it.
* Bienvenido/a de vuelta. (Welcome back.) This is a more direct and simple way to say it.
Other options:
* Me alegro de verte de nuevo. (I'm glad to see you again.) This is a more personal and friendly way to say it.
* Te extrañamos. (We missed you.) This is a very heartfelt and warm way to say it.
You can choose the best option based on your relationship with the person you are talking to and the situation.