"I am at home too."
Translation:Ja też jestem w domu.
To some extent the ability to judge the intended meaning is important. Sure, there are numerous sentences that might be understood in more than one way, sometimes contradictory („Motocykl wyprzedził autobus” anyone?).
What is, without a specified context, more probable? That you are at two places at once, or that you are at a similar place to the one another person is? Each one can try answering it oneself, but „Jestem też w domu” leans in meaning towards the former
It can be substituted with "to" in sentences built as "X is Y", where both X and Y are noun phrases. This is not such a phrase. Here, you just missed the verb and it shouldn't be an option.
Truth be told, "Ja też w domu" in everyday language would make some sense, if let's say you have a group conversation (a text one) and person A asks "Gdzie jesteście?" (Where are you?), person B says "Ja jestem w domu" (I'm at home) and person C says "Ja też w domu." But such 'short answers' often aren't grammatical sentences on their own, so without a very specific context they really shouldn't be accepted. I guess in English, person C could say "Also home" or something like that, which is not exactly a sentence.