I have run many dungeons this week and there have been players that have attended then left the run without talking. Nobody can report he players and the links send you all over the place with no availability to actually report the players. “The Ready Check” needs to have an option to put the players response in the chat so that others can report them.
You can right click on the character portrait; there should be a report option in that menu (that’s usually how I report players anyway - I rarely use the chat, because, like you said, most people don’t even talk while running instances).
Just to ask: Are you talking about normal dungeons or M+?
Leaving a run is not something you can report because it is not against the rules, with rare exceptions that cross the line to griefing. Leaving enough in Mythic+ can cause someone to be penalized by that is auto tracked by Blizzard as far as I know.
For other activities the penalty for leaving or being kicked is a debuff keeping them from joining another run for 30 minutes.
It is not fun to have another player leave a run, but it is not against the rules or something you can report.