MATLAB: 2D matrix or cell with groups to compare – different length groups, unknown number of groups

cell arrays

I've been creating a file for analyzing some image data. I need to be able to edit it each time I use it. It will take groups of slides to compare to one another. The groups will often be different lengths and the number of groups will change. Also I would like other users to be able to be able to use the code without needing to understand too much about how it works. I know that cell arrays can take variable lengths in the cells so I was trying to use them but I can't seem to get it right.
Here's an example of what I might be trying to compare: If I have 8 images numbered 1,2….8 I make groups of images I want to compare:
group1 = [1 2 3 4]group2 = [5 6 7 8]group3 = [3 6]
then I put them in a cell array:
compareImages = {group1;group2;group3}
But I can't seem to access the individual elements in the groups.
printG1 = compareImages{1}
works but
printG2 = compareImages{2}{1}
Gives the error message: Cell contents reference from a non-cell array object.
I would prefer to initialize the cell array in a single line so the user doesn't have to add and remove groups but rather the whole thing would be something like this:
imageGroups = {1 2 3 4; 5 6 7 8; 3 6}
This seems to work well if the groups are all the same length but not when they have different numbers of elements.
group1 = [1 2 3 4];group2 = [5 6 7 8];group3 = [3 6];A = num2cell(group1);B = num2cell(group2);C = num2cell(group3);imageGroups = {A;B;C};
printG2 = imageGroups{2}{1}
compareSlides = {{1 2 3}; {3 4}; {1 3}}; % initialize the whole array in a single line
printG2 = imageGroups{2}{1}