I'm not sure I understand the request. Are you asking if two umbrellas and two speed lights are sufficient to shoot a group of 10 people? Or if they'll work in a convention hall? Or your friend just needs photos of 10 separate cosplay individuals? Okay, some tips...
1. Look for a place to set up. I find this works well at weddings, events, conventions. Maybe it's a space of blank white wall or a hallway. But the idea is that it provides a decent space to shoot, some consistent lighting (so you're not having to fiddle with speed light settings and umbrella angles for every individual). To make it work, you need an assistant (one person watches over the setup and the other go gets the people you're shooting).
2. I find that cosplay shoots work better if you can work in some props. Maybe there is an expo hall with a backdrop you can use to shoot with. But without props or a backdrop, it's just a bunch of people in costume (and uninformed people look at it and go "oh, halloween party--right?").
3. Here's something that might work well depending upon the cosplay outfits and what I just said about the right backdrop/props....every convention center will have hallways used primarily for staff connecting the meeting rooms. Usually stark, concrete floors, scarred walls (i.e.: stains and skid marks). It's for food carts to go through or people to move in dais or carpets or screens or equipment). Try shooting in one of those--usually pretty quiet and often unused/vacant and you can get an interesting mood. Another good option for a conventional center is to go to the upper floors (usually the space that is vacant or used less b/c it's a hike to get there) and then put the group on the steps/stairs and shoot them that way. Also, I'm guessing that for a cosplay convention center, many of the meeting rooms go unused (it's not like a conference on fire safety or training or immigration law where there are dozens or hundreds of seminars and presentations going on) so slip in to a meeting room for 20 minutes and shoot away.
