Hey Aolas,

The answer to your question may not be satisfying, but it's what it is.

The command subjunctive is formed in a very specific way: you take the infinitive form of the verb and remove the "to". You do not use ANY other words (be, should, etc.).

I demand that you come home. (infinitive = to come, so subjunctive = come)
I insist that you buy dinner. (infinitive = to buy, so subjunctive = buy)
It is required that Frodo climb Mt. Doom. (infinitive = to climb, so subjunctive = climb)

Now, the only thing that can complicate this is voice. Every transitive verb has both an active and a passive infinitive:

Eat -- Active infinitive: to eat, passive infinitive: to be eaten.

So if you're trying to form the command subjunctive in the passive voice, you do include "be" (because that would still represent the infinitive without "to").

I demand that he be allowed to enter the castle. (passive infinitive = "to be allowed", command subjunctive: "be allowed")

In the sentence you gave, "come" is in the active voice (as a side note, "come" is not a transitive verb, meaning it can't take an object, thus it can't ever be in the passive voice), so we would not put the word "be" in front of "come".

Hope that helps!

-t