ON $*:TEXT:/^\w+ are you there\?$/iS:#: $IIF($1 ison $chan, MSG $chan $1 is here., MSG $chan $1 is not here.)
The above code works if you type in exactly as in your example.
This assumes that you have userlists enabled. Also, viewerlists can be a little... weird... it can take up to a minute for JOINS to show up in Twitch chat, and occasionally some names will randomly PART and then REJOIN within seconds.
Having said that, I second Fonic_Artes comment about it being rude to call out lurkers by name. Just a personal opinion, but it's a common one that you'll see on Twitch.