This code probably isn't doing what you think it is. I've commented it out below so I could run some code later in this answer.
% for i = (1:size(group))'
The size function called with 1 input returns a vector containing the size of the input in each dimension. When you call the colon operator : with one of the inputs being a vector (rather than a scalar) the operator ignores all elements after the first.
In addition, the colon operator creates a row vector. By transposing it you create a column vector. How does the for keyword handle iterating over a column vector? It uses this syntax from that documentation page: "valArray — Create a column vector, index, from subsequent columns of array valArray on each iteration. " So this loop is only ever going to run 1 iteration (unless group has 0 rows, in which case it won't run at all.)
If you want to iterate over all elements of an array I'd use the numel function instead. Compare v1 and v2 below.
c = {1, 2:3, 4:6, 7:10}
size(c)
v1 = 1:size(c)
numel(c)
v2 = 1:numel(c)
Of course you can perform arithmetic on numel in your loop.
v3 = 1:numel(c)-1
