It may be that even users outside the USA can participate in a USA class action. There's still a lot we don't know, but it seems Bitfinex intends to try to make users not affected by the theft settle for partial payments even though they are not insolvent and do not declare bankruptcy. That would be legally unprecedented and, it seems to me, legally impossible.

If Bitfinex concedes liability for the theft, they will get sued by the people whose funds were not stolen.

If Bitfinex contests liability for the theft, they will get sued by the people whose accounts were stolen.

I don't see Bitfinex getting out of this without a lawsuit from a significant fraction of their customers. A Court will have to determine whether or not they are liable for the theft. If so, they'll have to declare bankruptcy. If not, they cannot spread the losses.

I am not a lawyer. There might be something I'm missing. But nobody has told me what it could be.