The paragraph that speaks about the deal contains the sentence:
But she was not trusted, and so there was invoked a curse. Some curses are easier to cast when they bind yourself and others alike; Slytherin’s Parselmouth curse is an example of such.
There’s no statement that only the people who signed are binded.
The paragraph that speaks about the deal contains the sentence:
There’s no statement that only the people who signed are binded.