I do protest ! There are very few certain things in life ... one of these is the bipartition between "-en" and "-et" words in the swedish language. The word "grupp" is definitely an -en word ... ergo ....it should be "Är den sista gruppen?" and not .... "Är det sista gruppen".
It's essentially the same thing as the det är construction, where Swedish defaults to the neuter-gender option.
You could also say är det den sista gruppen? which makes this much clearer, but it's common to skip the definite article in this specific construction.