Well when there are a lot of people from the same fandom meeting up for a photoshoot it's a general thing that one person takes charge, typically it's the person that organized the shoot but not always. They can either set up poses or just ask for certain characters together, especially if they're part of a certain group in the show. Sometimes people will request a pose of certain characters and that will happen. Othertimes it's a more spontaneous thing. With big everyone is invited photoshoots everyone just uses their own camera.

A lot of time specific cosplayers will meet up to have a private shoot. A lot of time that is with either someone in the group who has a good camera and shoots well or someone they hired for the shoot. They tend to go and find a location or already have one in mind. After it is done the photos are usually shared online via facebook or email.

General rules might be lighting and location. You can't exactly have an amazing 'wow that look's like it just popped out of the series' photoshoot inside a convention center. It's better to go outside and find someplace or meet up with that sole-purpose. Definitely pay attention to who the characters are and how they would do certain things. And please pay attention to where you are unlike those hetalia cosplayers that caused a scandal a little while back.

Also I'm not some photograph/photoshoot pro so what I said is pretty open.

A regular conventions photoshoot could do to have the word 'photo' replaced with 'crap'.