Here is the code 

var inc = new GlideAggregate('incident');
inc.addEncodedQuery('active=true');
inc.addAggregate('COUNT', 'category');
inc.orderBy('category');
inc.query();

while(inc.next()){
gs.print(inc.category + " " + inc.getAggregate('COUNT', 'category'));

var number = new GlideRecord('incident');
number.addEncodedQuery('active=true^category=' + inc.category);
number.query();

while (number.next()){
gs.print(number.number);
}
}