I use the Preview app that came with my Mac. Photoshop works well (better even) too if you have it. I have it on my work computer so if I’m staying late I’ll sometimes sneak in five or ten minutes of groupshot building. My love of grouping is why I make all of my illustrations with transparent backgrounds 
It’s a pain when the image extends beyond the frame of the 400×600 transparent png option when you export the image. I have to kind of move the whole image around in part groupings, export each section as a separate piece, and build it again in Preview, against a blank transparent background with large enough dimensions.
For instance in this last image.
First. I have to create a wide enough canvas to paste all of the images onto. Preview has this annoying aspect where you can’t create just open a new image without copying something onto the clipboard. I usually just select a blank piece of my illustration, a tiny square of nothing, copy it, and open the selection in a new window.
Then I change the image size of that blank transparent square to something ridiculous I would have room to work with like 5000×5000 pixels or something.
From there you just open whatever images you want to paste onto that background, copy them, paste them, move them around and set them. Preview is also lacking in that once set, you can’t move layers around (neither laterally nor in terms of layer order), so I kind of have to plan out where I want to fit everybody before I get pasting.
In this instance I created four different base characters out of my existing Golden Choir convert image, and just pasted them and arranged them in that shape. Finally I made some changes to Ziva in HM3, saved the new file, copied, pasted, and arranged her over everything. You can do the same thing against a previously created background image like I did with Spectrum and 1960’s Spectru. I think I’ve made it sound more complicated than it really is, but it’s fairly simple.
