Just like having 15 people for a 25-man raid.
I'd just like to remind everyone that mentioning these threads in your local trade chat, to friends, guildies, in livestream chats, etc., it helps build the community here. There are constantly people cycling in, but there are also those of us that are done and leaving.
Keep the threads alive by advertising us!
Throw out some posts like this one to advertise
"Interested in Bronze, Silver, or Gold Challenge Mode runs? Check out the 'Cross Realm Challenge Mode Finder (5)' thread over on the Dungeons, Raids and Scenarios section of the Official WoW Forums."
That being said, for new folk.
Check out http://us.battle.net/wow/en/forum/topic/6863526914 for general Challenge Mode info, and Challenge Mode Gold run guides; or search for runs on YouTube to get a general idea of the instances.
General requirements are Buff consumables (Stat food, Flasks, Stat Potions), as well as Lesser Invisibility Potion / Invisibility Potion when pushing for Silvers/Gold, voice communication, and some time to dedicate to the runs. If you're pushing for Silver or Gold, it might take some time. Just be willing/able to give it your attention for a while.
When posting to look for a group, mention a few things.
Horde or Alliance
Times when you're available
Which instances / Medals you're looking for
Experience in Challenge Modes / other creditable things
An armory to the toon you plan to run with (If not posting from that toon)
It should look something like this:
Looking for all dungeons
0/9, but former HC raid experience
Feel free to add anyone who posts under this forum. Credits for this piece of writing goes to a US player.