There's more then one instance of its use. I think there are three places you would need to change.
I have mine to say Registered Users Since Jan 22 2001 on the main index page and on Who's OnLine it says just Registered User(s).
I did this by creating a new variable in Languages/online.pl $lang{'REGED_USERS_OnLine'} = "Registered User(s)"; Then in online.pl I changed the variable $lang{'REGED_USERS'} to $lang{'REGED_USERS_OnLine'}
In languages/wwwthreads.pl I changed $lang{'REGED_USERS'} = "Registered User(s)."; to read $lang{'REGED_USERS'} ="Registered Users Since Jan. 22, 2001.";
Brew
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