Found this one with a client recently.
Can't move computer from one group to another
If your computers have special characters in their names that are not standard (like "_"), you won't be able to move said computer to another group when doing it from the "edit" button.
There's an easy solution to this.
Go to the "Network" section, choose "Grid view".There, you can simply select the computers you want to move and do so by simply right-clicking -> "move to group" -> "name_of_group".
(You should really not use special characters in your computer names, though.)