Challenging

Here are the criterias for making it a guildrun;
3/5 - dungeons
8/10 - 10 player raid, 10 player BG
20/25 - 25 player raid, 25 player BG
10/40 - 40 player raid
12/15 - 15 player BG
2/2, 3/3, 5/5 - Arenas
(edit: updated to reflect the 4.1 changes)

Those limits have actually been lowered in 4.1 for 5-man content.

To earn Guild Dungeon Challenge credit, the 5-player party must be composed of at least 3 guild members. Raid and Rated Battleground Challenges require that 8 out of 10 players be guild members, while 20 guild members are required to earn credit for 25-player raids. To earn gold for Guild Challenge rewards, a guild must be level 5 or higher

http://us.battle.net/wow/en/blog/2534919