Yeah...how will the whole RP begin? It's sort of important to know when planning the first post.
Right, Here's how this is going to work...
Everyone from the Resistance will have a Flashback scene mentioning how they were pulled in. Since it was a dimensional pull that eats away at the dimensional barrier of their home universes, it can be whatever makes sense for that universe. For example, if they have portals, you'll find they have one unusual portal that sucks them in. If they have teleporters, there will be a malfunction that causes the Teleporter to teleport them here. Etc.
After that, they may either choose to be there for a while and it'd be a "Cut to later" between past and present, or they may choose to continue directly afterwards and be a new guest in this world.
If the former, they're in the resistance base with the rest of the Crew, or out scouting or doing recon, whatever you like them to be doing at present.
If the Latter, they'll probably land somewhere not too far from the base. It maybe an empty region of dead-lands, or it maybe an enemy territory.
Either way, their presence will be detected by the resistance in the base, in which case their retrieval becomes priority. If the Empire gets to them before they do, after all, they'll either convince them to join wrong side, or just outright attempt to kill them. I will leave it to the other members to brief any newcomers.
-------------------------------
The Stars, generally the same thing as the Resistance regarding having a Flashback scene detailing how they got here. However in their case it will most likely be the latter, and you may consider them as a "Late-arriving third party". They won't wind up exactly in the same spot, but more likely within the same area...Like being in the same city but in different spots of the city. In which case their first objective would likely be to find each other, and once they do, their first mission and general explanation of the situation will be given to them, specifically by their Guardian.
------------------------------
With the Empire, The Flashback scene is optional. They may choose to have a Flash-back scene if they like, but it will cut to the present with them in their cities. For the City rulers, give a good idea of what the City is like and the minions within it, as well as what the ruler is doing at the moment. For members of the Covenant, They will probably be united in the Capital, undergoing a meeting. One particular member of the Empire, being made by Windpipe, will be with the Covenant at the time, to give them some...information to work with, regarding the Resistance and the Stars.
------------------------------
Finally, the Neutrals...Basically can make their debut however they wish...EXCEPT the Doctors. ...I'll be discussing how we're going to be doing their debut via PM.
-----------------------------
I'd think the Resistance is being vastly outnumbered by this point.
The Resistance is outnumbered even if there was just one City-ruler. The armies of the Covenant alone outnumber them. ...That's what makes interesting. The Resistance CAN'T approach this conflict directly, or they will lose. They have to be clever, work outside the box, and use Stealth and recon in addition to force to do this right.
Look at any fiction where it's resistance against a global power, and the same holds true for them. Outnumbered, but not exactly outmatched.
That being said, The resistance has NPCs of their own to help them out, So it's not exactly "7 Vs. 7 Billion" like it seems.
They will most likely help the Stars instead of the Resistance. Though, I'm not sure about Daniel.
This is true. The resistance are the good guys, yes, but they are soldiers at the end of the day, and are fighting to defeat, and most likely kill. That's not how the Doctor works; to join their side is to essentially become the War Doctor again.