Well, I'm going through this myself, so I can give you some advice.
On determining what to cosplay from, people can nominate series and then you could vote. Or you can do what the Cake on a Plane cosplay group did (or tried to do) this time around, where we just started listing any series we could do to broaden our perspective, and then we all listed our top three choices. From there you can see if there is any single series that most people would like to do. Ours ended up being Strawberry Panic.
We haven't come across the problem of two people wanting to cosplay the same character. But if one person has another character in mind than they should just step down and do that character instead. Or you can make them both be different characters. Or you can decide it through rock-paper-scissors. As long as everyone has a good attitude about it things should work out.
Another tip is make sure that there is communication within the group. I don't know who you're trying to do this with, but CoaP is organized over the forums, and it's difficult to make important decisions without everyone there to add their input. Communication is vital. That's how our group broke down last time; people just stopped posting.
That's all I have for now. Good luck!
[Btw, if you change your mind and don't think you're up for the job of leading this group you speak of but still want to be in a cosplay group, feel free to join Cake on a Plane.
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