If you look at the documentation the input variable whichstats can be a function handle. So for your example of the population standard deviation we can do the following:
x = rand(50,1); % Make some random data.
grpVar = logical([ones(25,1);zeros(25,1)]); % Split data into 2 groups
popStd = @(y) std(y,1); % make a function handle that computer pop. std.
myStats = grpstats(x,grpVar,popStd);
Using this method, you can make your own statistical functions. Also the list of stats given in the doc is not exhaustive. For example:
myKurt = grpstats(x,grpVar,'kurtosis');
returns the kurtosis of the groups.
