What happens if a group of witches/wizards swear an Unbreakable Vow and one defies it?

Question

Supposing a large group of witches/wizards swear an Unbreakable Vow (an example for this question, to keep a secret) and one of them chooses to defy it later on and dies as a result- is the spell effectively 'broken' or are the others still bound by it?

Answer 1

Yes, the spell is "broken" because...

I don't think there is any evidence that more than two witches or wizards can swear an Unbreakable Vow. It seems that two people swear the vow, with another person acting as the officiant or Bonder:

“Certainly, Narcissa, I shall make the Unbreakable Vow,” he said quietly. “Perhaps your sister will consent to be our Bonder.”

Harry Potter and the Half-Blood Prince

Indeed, in the only example we have, two people make the vow:

“You will need your wand, Bellatrix,” said Snape coldly. She drew it, still looking astonished. “And you will need to move a little closer,” he said. She stepped forward so that she stood over them, and placed the tip of her wand on their linked hands.

Harry Potter and the Half-Blood Prince

I suspect that multiple people must make multiple Vows, and thus breaking one has no effect on the others.

Answer 2

No, the Vow will not be broken.

Firstly, I agree with @Nu'Daq's comment to the question. While there is no evidence to prove that an Unbreakable Vow can be between more than two people, one example is hardly enough evidence to prove that an Unbreakable Vow cannot be between more than two people.

Secondly, I believe that if it is possible for more than two people to be bound by an Unbreakable Vow, one person breaking the Vow will not release the others. Here's my reasoning for this:

She stepped forward so that she stood over them, and placed the tip of her wand on their linked hands.
Narcissa spoke. “Will you, Severus, watch over my son, Draco, as he attempts to fulfill the Dark Lord’s wishes?”
“I will,” said Snape.
A thin tongue of brilliant flame issued from the wand and wound its way around their hands like a red-hot wire...
_{Harry Potter and the Half Blood Prince, Chapter 2: Spinner's End}

As we can clearly see, the Vow is made from Snape to Narcissa. Snape is bound to Narcissa by what he has vowed.

Consider the hypothetical situation where Narcissa wants Vincent Crabbe and Gregory Goyle to watch Draco's back and stand by him no matter what. Assuming Crabbe and Goyle agree to make an Unbreakable Vow on this matter, Narcissa can link hands with both of them at the same time. Since she requires the same vows from both Crabbe and Goyle, the same actions can be performed (as described in the quote above except with Crabbe and Goyle each linking up with one of Narcissa's hands), with Narcissa asking the question (For example, "Will you, Vincent, and you, Gregory, stand by my son, Draco, through thick and thin irrespective of the circumstances or situation?") and Crabbe and Goyle both giving their answers ("I will") simultaneously.

If the spell is performed correctly, Crabbe and Goyle should both be bound to Narcissa by the same Vow. However, there is no link between Crabbe and Goyle in regards to this Vow. One thin tongue of flame that issue from the Bonder's wand will wrap Narcissa and Crabbe, while another tongue of flame will separately wrap Narcissa and Goyle. Narcissa is the focal point. They are each bound to her. If Goyle then breaks the Vow (and dies), Crabbe is not affected in any way since his Vow is directly to Narcissa. Crabbe is still bound to Narcissa by this Vow. Each person has only made the vow as to his own actions. They haven't vowed collectively as a group.

Having said that, if the situation entailed a group to vow together ("We will"), I believe one of the group breaking the Vow would be tantamount to the entire group breaking the Vow, and hence they would all perish.