if it were active voice it would be" the gardener watered the plants." with passive voice we can emphasize the plants in this case rather than the Gardner itself. i hope this helps!
A subject is the word, usually in a form of nouns, that is being talked about in the sentence. A subject can either be simple or compound. In the given sentence above, the subject is the word "we". And "we" is the direct doer of the action word "invite".
