Since in English, 'welcome home' would only apply to someone who lives there and wouldn't apply to a guest (an example could be: a young man or woman who went off to college and just came back for a winter break, so the mother would say "welcome home" with open arms).
If that's the case (implying it's the person's home that he or she is being welcomed back to) than you could say "bem-vindo/a em casa", right? ...porque implica que a pessoa que é bem-vinda mora lá.
**I had to edit because I thought of a separate situation. The expression "welcome home" is also applied to a hometown, neighborhood, family situation/locale, or even social club/bar/restaurant or job that a person used to frequent or work at...so in this case the idea of home does not work with casa...so I guess it depends on the context.
In this case I want to send an email to some Portuguese people I met in Barcelona and I'd like to open it by saying "welcome home" in Portuguese. So if I put sejam bem-vindos a casa would that be correct?