Most characters will surely already know one another, having all gone to school together and presumably been aware that everybody else is going on the trip. I'm not sure about Lydia, but I'll throw Johann in shortly after the RP starts. Do we have an idea for what our first actual little adventure will be? Again, I wasn't in the last incarnation of this so I don't know.
I like Primez's general outline. It almost suits the narrative about wizarding Germany to have the group consciously avoid Germany at at least the start of their European adventures. Perhaps events conspire to take them there or they simply go there a little later in their trip. Or, they might warily plan on going at the start of the trip and get into trouble with authority - their magical Visas aren't valid (or whatever) and they basically get deported. Of course, they could always try to return later to fit the story. Since we're kind of going to be making up the wizarding world, beyond muggle events and geography, we can simply make up the politics of the wizarding governance of nations to suit the story we want to tell - to that end, it might be worth people with a preferred interpretation of one particular nation to throw ideas out as and when they occur.
Incidentally, if we're to tie any of our adventures to those NPCs and PCs that're tied to Beauxbatons, it's worth noting that Beauxbatons is located in the Pyrenees in the South-East of France and nowhere near Paris.
Also thinking of throwing together a last-minute Slytherin to keep Grant company. Was thinking perhaps an ex with whom he's now close friends (to the immense displeasure of his later conquests).