I know that I've seen definitions of the Jaccard index like the one that you are referring to however, those were constructed incorrectly by people who don't know how to use binary or statistics. This is obvious because you should only achieve a maximum of 100% because that's realistically the highest percentage of organisms that could be shared between two communities.
Instead let me suggest this form of Jaccard's index
Index = a / (a+b+c)
Where a = the total number of species in common between site 1 and site 2
b = the total number of species in site 1 but not site 2
c = the total number of species in site 2 but not site 1
Notice at first a+b+c just seems like the total sum of species present between the two sites but I assure you it's not.
Let's think of two communities with 50 species each which so happen to be the same 50 species.
a = 50
b = 0
c = 0 therefore a +b +c = 50 and 50/50 = 1 or 100%. If you had simply totaled the number of species you would have gotten 50+50 =100 and 50/100 = .5 and that would've been wrong.
Also, this equation is adjusted for the example you gave
If there are 10 species in community 1 and 2 with 8 in common
a = 8
b = 2
and c = 2
8 / (8+2+2) = 8/12 = 66.667% and not over 100%
Hope that helps