Right now, in order to count as a guild group, four out of the five party members of that team must be from your guild. With these changes, a guild will be able to take three members along and still qualify for guild experience, albeit at a slower rate. What does this mean? Mainly that smaller guilds have more of a chance to reach that experience cap that larger guilds have been hitting without fail every day. The experience gain is only 50% of the normal rate, but at least doing dungeons with your guild members in smaller numbers will actually mean something.
On the other end of the spectrum, if all five players in a dungeon run are from the same guild, that group now earns a staggering 125% guild experience. Why the change? It seems as though Blizzard is trying to encourage people to group with their guild members, by offering more incentives for doing so. This is all well and good, but if more guilds are running dungeon groups, this means less people using the dungeon finder and longer queue times for those who have to wait. We'll have to wait and see what the results from these changes look like once the patch hits live.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from leveling up a new goblin or worgen to breaking news and strategies on endgame play.