>> A = 1:4; % the group enumeration
>> N = 3; % the number of rows in each group
>> reshape(repmat(A,N,1),[],1)
ans =
1
1
1
2
2
2
3
3
3
4
4
4
You can then simply allocate those values to any column of your matrix, here are three options (you did not specify how you want this to be allocated):
X(:,1) = reshape(repmat(A,N,1),[],1); % into existing first column
X(:,end+1) = reshape(repmat(A,N,1),[],1); % into a new trailing column
X = [reshape(repmat(A,N,1),[],1), X]; % into a new first column
