We have a large whiteboard in one of our conference room, and there is a dent about 3 inches long and 2 inches wide. We were thinking that maybe using spackle or something similar to fill in the dent and then paint over it with whiteboard paint.
Sand the area around the dent, until the white part is gone, usually down to the metal, then use bondo or other automotive body filler and fill in the dent. Feather the bondo out and sand it smooth. Then paint your whiteboard paint over that. YouTube should have plenty of videos showing a proper use of bondo or body filler. Lots of work but will be a professional looking fix.
The easiest solution might be to dismount the board and carefully fill the dent from behind the writing surface with material similar to the rest of the "underlayment".
Another option would be to obtain a piece of the same type of whiteboard, whether purchased or salvaged from another board, cut out a rectangle around the damaged area, and create a precisely measured patch from the additional piece. There might still be a slight border around the patch requiring a bit of fine sanding and covering with varnish (or clear nail polish).
Spackling would almost certainly fall out and would create an uneven writing surface until it broke loose.