Hi, Fatima. Which situation do you want to say it in? That what I've first thought of once I read 'Welcome Back' is "잘 오셨어요(in honorifics)", it could be "잘 왔어요(in honorifics, but more informal)" or "잘 왔어(between close friends)". One expression carries different meanings in different situations and there is a little big gap between written Korean and spoken Korean. If some workers returns to work after they work overseas for a while, they would see a picket saying '돌아오신 것을 환영합니다', it's kind of in written Korean style. If you're gonna say it in return for someone saying '(I'm) Back.' on an instant messenger for instance, one of those three things would be proper. 오셨어요?, 왔어요?, 왔어? are also proper to say in the situation.

돌아온 것을 환영해요。 돌아오신 것을 환영합니다。（honorific form）