I agree with you. Besides, I don't see any 100 in the original sentence.
So it may be 8 %, 80 % or 800 %
But last year the town did see an eight-fold increase in membership to Promo. "We've only been tolerated because we've remained silent.
...la ville a bien vu 8 fois plus d'adhésion? maybe
Yes... whatever the increase usually is = the percent increase (not necessarily 100%!)
eight fold = the increase x 8
For instance, if the usual increase is 10 people, then the 'eightfold increase' would be 80 people.
If it's 100 people, then it's 800 people extra.
I'm not sure what you mean hereI think this is overcomplicating it - if you are talking about the membership of something increasing eightfold, I would say it just means that there are now eight times as many members of whatever it is, as there were before or as there are usually. ie if the event usually attracts 100 people, this time it attracted 800 people.
To have the meaning you are giving it, in my opinion, it should say : "last year the town saw an eight-fold increase in the increase in membership to Promo"...
you said the multiplication by 8 should be applied to the usual size of the annual increase in membership; I think it should simply be applied to the usual number of members; anyway, I guess it's fairly irrelevant for the purposes of translation - eight-fold just mean multiplied by eight either way (which is the same as an increase of 800%, as Keith said)