x=[rand(10,1) randi(3,10,1)];
% this is the sum for groups 1,2,3
[groupsums,groups] = groupsummary(x(:,1),x(:,2),'sum')
% this is what you described, the sum of the cumulative sum
[groupsums,groups]=groupsummary(x(:,1),x(:,2),@(x)sum(cumsum(x)))
% here's just the cumsum:
x=sortrows(x,2); % sorting is not necessary, just makes it easier to visualize
groupcumsums=grouptransform(x(:,1),x(:,2),@(x)cumsum(x));
[x groupcumsums]