Default nightbot variables don’t have any random things to do what you need.
However using rtainc’s customapi you can make a command that pulls a random user from chat.
In this case the command you want could be done like so (just copy and paste the commands):
!commands add !hook -ul=mod $(urlfetch https://api.rtainc.co/twitch/channels/$(channel)/viewers/random?format=+get+over+here!)
If you don’t want it to be mod only you can modify -ul=mod, you don’t need to touch anything else unless you want to change the format after