What's the best way to find out if they are interested? Sometimes it's hard to tell if someone is interested in more or less photos. (ok, usually it's easier to tell if they want less...) Usually I avoid asking people to do photo shoots because I think it will inconvenience them too much.
Please just ask! If the person is a relatively new cosplayer like myself, they may be a bit surprised by the request, but I would certainly be flattered. I am thrilled whenever someone thinks highly enough of my costume to ask to photograph it. Unless I'm running late for something, I would be totally happy to stop for pics. As for the group thing, my anime club tends to all cosplay from the same series. If someone wants to single out one of the members, we're totally cool with it. We actually often congratulate that member when they return to the group, since we all watched each other put the effort into our outfits.
To answer, previous questions:
Unless I have somewhere to be, asking to move to another location would be perfectly fine with me. To avoid the "creepyness" factor, I would feel a lot more comfortable if the photographer gave a specific destination when they ask, like "it's really crowded here, would it be alright with you if we move to the fountain over there/the gazebo on the second floor?" That, and I might ask to have a friend come with me to hold my bags/make sure my costume is on straight.
And as some people have already said, I would love it if the photographer would give me posing tips. I can't see through the lens like you can, let me know what I can do to improve the shot! Also, even if you aren't familiar with the character please let the cosplayer know if something looks off- bra strap showing, roots visible under the wig, lipstick marks on teeth. If anything, I'd be relieved that you let me know!