This question made me think about the structure of the sentence.
I'm familiar with the expression 'Enter Michael'/'Exit John' to represent Michael's or John's entry or exit, respectively, to a dramatic stage, and its metaphorical use.
Is this a real sentence, or a sentence fragment? Either way, should John be treated as the subject of the sentence, and put in the nominative case? I realise that this makes little practical difference as English words are not typically declined based on case, and "exit him" or "Enter I" sounds wrong either way. However I am curious.
Initially I had thought that "exit" might be the subjunctive, and that the phrase was equivalent to "Let John exit". But the plural of "exit" in stage directions is "exeunt", and both "exit" and "exeunt" are present indicative in Latin.