Eh, just say that Arex and M'Ress were aboard but not on the bridge crew yet, the same way Chekov was before S2. Presumably Uhura has a communications staff somewhere on the ship, and maybe Arex is serving in auxiliary control or something.It has Kevin Riley (S1) and Chekov (S2-3) and Arex and M’Ress (TAS) all aboard, which kind of throws me for a loop. Sort of the Chekov-in-“Space Seed” issue squared. But I suppose rather than imagining people joining and leaving and rejoining and leaving again, the intent would be that they were all “really” along for the entire Mission, whether or not you saw them in a given story (or massive chunk of stories).