The majority of the guild players were around Level 30 and were relatively inexperienced, which meant they weren't able to actually go to Thousand Waves Lakes and fight in the leveling area. However, they were good enough to stay on watch on the road from Congee City to Thousand Waves Lake.
Most of the players in the tenth server were around Level 30. If they hid their guild tag, it would be impossible to recognize who they were if they stuck within a crowd. As for those Level 35 and above players who had just joined, Ye Xiu didn't maneuver them because he knew that there were spies hidden amongst them. He couldn't be certain that those players who were around Level 30 weren't spies, but the risk for that was much less. Even more so, Ye Xiu wasn't ordering every member of the guild to move. Instead, he only picked a handful of them, asking them to take note of certain players.
If a spy got in, then he could only blame his bad luck.