Except, you can not queue an 8 man team for regular WZs. Two 4 man teams from the same guild can queue at once, but it is just as likely that they will end up with 4 PuGs instead of their other 4 man guild team.
If you did run into 8 members from the same guild, then it probably means that there were extremely few people queuing up to PvP.
it's not hard to do they just Que up at the same time, as said several times in this thread. Let's say your in a guild you got 8 guys ready to go, you make your 2 groups and que at the same time. The odds are relatively high that they will be tossed on the same team because it's faster. The "random match making" that the game has isn't very random it grabs what is the fastest and easiest in the order it was qued. It's faster to grab two 4 man groups and toss some extra's on the other side then to split the two 4 man teams into two seperate games and then wait to find even more people.