@WesternAngel- Usually there are two methods. First you do the group shot within heromachine its self, by creating all the characters together in one image the same way you’d make a single character. You can use characters you’ve already made by clicking a little tick box at the bottom of the “Load Character” menu which says “keep currently loaded items, this will keep any character you’re currently working on and load another into the same save. But if doing that make sure that each character is positioned where you want them in relation to each other before loading them into the same file, because keeping track of items as you move them into position all in one place gets messy, much easier to move by item when the characters are in individual saves. Or secondly, if you’ve already made the characters individually and exported them, use an external editing software like photoshop or gimp, even word has rudimentary picture editing that can be used. Saves a bit of time.
Hope this helps.