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)
Overall, it'll probably look something like this,
2/9 Gold - 1/9 Silver - 6/9 Bronze & 2400 Arena exp & 2/16H exp
Horde Protection Warrior looking to do:
Gold Mogu'shan Palace
Gold Gate of the Setting Sun
Silver Siege of Niuzao Temple
Bronze Scarlet Halls
I can run any Mon-Fri 5pm-midnight EST
Add my BTag, Words#1111
Don't be afraid to add people from the thread and make your own group! They won't bite, I promise.
After all, someone has to be the guy or gal to start invites.
Good luck to those still finishing up, and to those here who are just starting.
If you think anything should be added to the OP, mention it!
Be sure not to spam post LFG. If someone wants to find you, they can look through the thread. You don't need to post once on every page (.. or twice.)
On the same note, if you have completed your challenge modes, or just do not want more people to add you, please edit your posts so that we know. Simply write "n/a" or "full/done."
If you're posting a LFM, try to remember to edit the post whenever you fill, to avoid mix-ups. And be sure to clarify which instance, medal, and faction.