I can see this information when I initially connect, but is there a command I can run to see an updated list?
You can issue the command
Exactly how you'd do it and exactly how the output is displayed depends on the client; I'm not familiar with irssi so can't really comment on that.
For other IRC commands, see for example technerd.net's summary of IRC commands. Note that many of those may very well be server-dependent.
in the whichever channel you want a list of users for. This is the IRC command to do what you want.