One view the list of users who are currently in a chatroom. Can one view who visited a chatroom in the past (past = before they left the chatroom), and if so, how?
Well, not really unless there is something that record such events.
There are some chatbots that do it, then you can execute a command that will show all visitors recorded by the bot.
For example see this in my chat room.
Also, you can manually browse to
/rooms/pingable/[room number] to get list of pingable users which means any user who visited in the last two weeks, assuming they have at least one message in the past. (Or currently in the room.)