My obedience dog moves his feet and scoots sideways during group sit stays. This started after another dog went after him. Any suggestions would be appreciated.

How long ago did the dog go after him? You may be wise to simply take a break from training for a while and just do some other activities.

I'd also recommend not doing any group sit stays at all if you are still having the same issue. He's probably nervous and you won't get him over it by continuing to put him in a situation that makes him worried. I might find the distance that he can ignore the other dogs, and place him there for a very short time with lots of rewards in place. I would not be working on duration, distance and close proximity to dogs all at the same time. If this was my dog I'd be practicing all kinds of sit and down stays on his own, far enough away from other dogs that he was not concerned. If he knows touch pads, you may also want to have him place his front feet on a touch pad so you can clearly tell him "touch" if he moves his feet.

I would not get in a huge hurry because in a trial situation where you are possibly more nervous he is likely to show a much more drastic reaction, he may learn that getting up and leaving is the way to be safe.

Some dogs never get over being attacked or scared by another dog, unfortunately.

Cindy