Well, not EVERYONE will be joining EN81, as it can be either conquest or revolt and most alliances are specialized on either one of those. Would be awesome if they did a double-world-release as someone proposed earlier, but should only one of the 2 be released and another months' long wait for the other one to follow, some might get a bit aggitated.
As far as "jump in the second the world opens" or "wait for a while and join late" goes I think every alliance has their own strategy regarding that and is actually irrelevant in the context of the world opening. Late starters just have to keep an eye on the world, as with such a long gap between worlds the world might witness a flood of players and might fill up faster than the late-joiners might anticipate.