Vastly differing XP is expected even with a steady party
First, yes, there's no problem with XP being completely divergent for different PCs. The XP system is weighted heavily toward individual experience (marking XP on a miss varies a lot, and generally outweighs marking XP for the End of Session move), making it normal for XP and levels to be different between PCs fairly quickly. And if any PC makes it to the end of level 10, it is normal to have one or more 1st-level PC adventuring with high-level ones.
And because there's no such thing as scaling challenges to party level, there's simply no problem with differing PC levels and nothing for the GM to do about it.
In practice, new players join pretty smoothly, and still get a say
In my experience, players joining after session 1 is less of a problem than it seems like on paper.
Part of the way worldbuilding works in Dungeon World is that it's collaborative, but the players aren't supposed to need to notice that they're contributing. The GM is just saying things and asking the occasional question, but the process weaves an illusion that the GM already had most of a world and campaign written out.
As a result, you're mostly off the hook for worrying about who gets to contribute to the worldbuilding: the players probably won't notice the difference anyway.
Additionally, a new player joining is a dynamic that makes integrating them into an existing game of Dungeon World easier than you might think. Most of the time they will be playing a different class than anyone else, giving them a domain of expertise that is unique to them, which you can keep asking them questions about to establish worldbuilding details. The initial party does get a slightly bigger say in the broad strokes of the world, but this is not a problem in my experience — a player joining an existing game doesn't expect to have a blank slate world to enter, because they are joining an existing game. To the contrary, they expect a world already in motion.
On the GM's side, just ask questions and use the answers as you would with any of the players, and keep weaving in things that you didn't know before, regardless of who it comes from, as per usual Dungeon World GMing after the first session. Certainly, the initial session will have set up some details of your second-session prep (fronts, etc.), but those are things that the players aren't really supposed to know they had input into, so new players don't really miss out. And because you're still playing to find out what happens, they will have input during the normal processes of sessions after the first, anyway.
Bonds are a small sticking point, but YMMV
Bonds are one of those bits of Dungeon World that are amazing and central for some play groups, and annoying and ignored by others. By and large, my group has been pretty “meh” about Bonds and they haven't influenced our games much. Yes, they're a nice way to create connections… but if the players just ignore them later, they might as well not exist.
The upshot, is that I an say from experience that Bonds not being all perfect between the PCs isn't actually a showstopper. You an bring in a new PC, and have them fill out one or more of their Bonds to find out “hey, how do you know these people already?”, and you can have the existing PCs voluntarily fill out any empty Bonds with the new PC.
But in the end, I haven't found this to be necessary, and the awkwardness on paper doesn't really translate into actual awkwardness in play. Bonds are simply not central enough to the game to “break” anything when they're just kind of haphazardly handled. You can get by just shoehorning in the new PC's Bonds and really not worrying about it. In effect:
- If Bonds are unimportant to the party, sloppy new-PC Bond connections won't be a drag anyway;
- If Bonds are important to the party, original Bonds will soon be resolve and open up Bond slots that players will start connecting to the new PC anyway, organically.
Either way, just get that new PC in there quickly and don't worry about the Bonds working perfectly or "fairly", because they'll work themselves out organically either way, by being ignored, or by being dynamic.