When you type !maps, a popup will be shown to you with every map on the server. No need to edit any maplist.txt or anything.
Admins change type !changemap and then a menu will pop up with all the maps, and then they can chose which map they want to go to
for admins, if you have groput_auth set up anyone with map_admin flag has access. Also, you can add specific steamids, and by default anyon ein mani's clients.txt has admin.