Not sure if this is an exploit but definitely a bug of some sort and fun to mess around with.
Now while you're in a raid with assist, let's say waiting for WG to start. When people get invited and get assist, if you right click them before they get assist and the little box that pops up doesn't close, you will still see remove.
Now you can just wait and remove them a few second's later.
It's pretty fun to do to piss someone off especially if they keep getting invited back.
Have fun with it.
Just trying to contribute.
Edit: Sorry if it's a repost. I searched "how to remove people who have assist." and didn't get anything.