There's usually a six year limitation period for what you can claim. As far as I can see, there is no right to charge a joining fee anywhere. So maybe a £1,000 per member as a starting point.
But in Iain Mitchell KC's opinion (
link here), he raises the question of unjust enrichment, which in turn might mean that no limitation period applies. It's a complicated question that he leaves open in his opinion, but it could mean it's possible to go back to 1997/98.
You'll see at the end of his opinion he urged the parties to try to sort things out by discussing them. All of us involved in Weighted Voting agreed and we tried. We've particiapted in many member calls and events. Unfortunately, all attempts to open more formal talks with the Nominet board have been declined.