What is the best way to show all of the users that are currently accessing the file that I'm in? Is it possible to display all of the users accessing the current file in a lay-out?

IMHO the best way is to use the FMS Remote Admin tool. It displays lists of current users for each hosted file.

There is no "native" way to do determine the current users or get them displayed in a layout: no function exists that lists account names (I imagine something like that would be a major security concern).

A user tracking system could be created however. It could be a simple as adding code to the startup script that creates a timestamped record in a "Log" table, and with code in the shutdown script that records when they close the window. However it won't necessarily be accurate because it will only track those users that have accessed the file in a way that causes the startup and shutdown scripts to run: it's possible to use files in ways that avoid this.

For example a file can be open hidden and it's data shared with another database (say, a contact database whose data gets used on many different solutions). In this case because the Contact database's window hasn't yet been opened so the startup script won't have run. Would this be considered being accessed by the user?