What happened to the good old Torrents?
The network isn't illegal or anything, it doesn't cost anything and if you want to share that 2 GB of SLR pictures with 108 other people, it's about the easiest thing to set up. Well, that only means that someone has to explain the setup of a Torrent client to all of them, whereas the geek in the crowd can create the torrent.
It worked for me, the 1.5 GB of pictures and the 100+ other people.
Dropbox on the other hand is totally great, I use it myself, buuut... it has some downsides: The 2 GB limit, the traffic limit (100klicks on that 1.5 GB zip? - bam, public links temporarily disabled), and if you want to create a shared folder, you have to pass e-mail invites to all the others, who have to accept the mail, sign up and install the software. For ~5 people it's easy. For 20+ it can become a pain in the neck.
EDIT: As it turns out, there's a web-based torrent client called BitLet, so basically there now only needs to be one geek in a group to set up a torrent, the other guys can use the webapp. Nice.