In the first meeting of Dumbledore's Army at the Hog's Head Hermione asks the students present to sign a parchment, which they all do. The parchment serves as a magical contract:
When the last person — Zacharias — had signed, Hermione took the parchment back and slipped it carefully into her bag. There was an odd feeling in the group now. It was as though they had just signed some kind of contract. ~ Harry Potter and the Order of the Phoenix
The parchment had a jinx on it, whose purpose was to serve as a warning they were betrayed:
Hermione’s eyes slid rapidly down the notice. Her expression became stony. “Someone must have blabbed to her!” Ron said angrily. ... “No, they can’t have done because I put a jinx on that piece of parchment we all signed,” said Hermione grimly. “Believe me, if anyone’s run off and told Umbridge, we’ll know exactly who they are and they will really regret it.” “What’ll happen to them?” said Ron eagerly. “Well, put it this way,” said Hermione, “it’ll make Eloise Midgen’s acne look like a couple of cute freckles.” ~ Harry Potter and the Order of the Phoenix
As we know, when Marietta betrayed the DA she'd SNEAK written all over her face among other unpleasant cosmetical changes.
I had the impression that a few people in the first meeting weren't that thrilled by the idea of the DA and only joined because of peer pressure, like Zacharias Smith, Ernie and Marietta, who was clearly intimidated by Umbridge and wasn't really secret group material. Why not allow people like her to leave the group, warning them only that if they'll disclose the secret they'll be jinxed?
Wouldn't it have been better tactics to tell the DA members about the jinx, to deter them from breaking the magical contract as it is done in the case of the unbreakable vow and the one that's placed on the Goblet of Fire?
After all, Hermione must have studied magical contracts and the jinxes connected to them to do her parchment jinx to get an idea how it's usually done.