Good luck with that one. You can try cloning background around the outer subjects, but that will be very difficult around such edges. Making the group a selection will help, so that you can clone around them without any effect on the actual group. Of course, if you are selecting the outer subjects anyway, you could just go the whole hog and select the whole group, cut them out, then place them on a clean background. Either way, you will need to be careful with the shadows to keep it looking real.
I have to ask, but why did you shoot it that way in the first place? It must have been pretty obvious that they were well off the background on both sides. As usual, the answer is to shoot it right and not just think "it doesn't matter, I can fix it in post later".
You could have unhooked the background roll and stood it on end at one side of the room, then run it across the whole width of the room. It is wide enough to still give enough height as a background. You would need to tear some off one end, to place on the floor of course. By curling up the back of the piece on the floor you would minimise the post work to just needing to blend out the join between the papers.
Or take a length and hang it vertically alongside the background stands you were using. Again you will have a join to clone out, it just comes down to which method is easier to hold the papers with.
Another method, if you couldn't do that with the background roll, would have been to take shots of the half the group at a time, so they would fit on the roll as it stands. Then simply take the two shots and combine them in post. You would need to select around the edges of the groups in order to layer one over the other slightly whilst keeping the background behind both, but it would be a much simpler job than you are left with now.
I know that such suggestions for shooting are too late for this shot, but may give you some thoughts for next time you have such a large group. It may be worth purchasing a large fabric background that can cover the extra width, whilst still giving enough for the floor, all in one piece. That would save any messing about both in setting up the shot and in post processing.
