I think that it's likely that after whatever skirmish may happen in Woodbury, that the zombie hordes will end up taking out whatever survivors remain in that town. I wouldn't be surprised if during their attack if some zombies get in the middle.
I think that is a strong possibility; walkers are already around the town, and if sound attracts them, Rick's group probably leads more in the direction of Woodbury. Once the battle begins, I can see Rick's group deliberately leading walkers into the town to act as not-so-human shields from Woodbury bullets and arrows. The town will be tossed into chaos, which--in theory--should allow Rick's group more time to search for Glenn & Maggie....
...but the preview seems to suggest Glenn and Maggie will be threatened to force Rick's group into a quick surrender.