Code:
on me:*:join:#:{ hadd -m counter # $nick(#,0) }
on me:*:part:#:{
if $hget(counter,#) { .hdel counter # }
}
on *!:join:#:{ hinc -m counter # }
on *!:part:#:{ hdec -m counter # }

$hget(counter,channel) to access it smile