There is an error in your example script, the correct syntax for this event handler is will be like this: on <level>:JOIN:<#[,#]>:{commands}

If you want this message to be visible to all users join the channel, to then the code might look like this:
Code
on *:JOIN:#:{
  .msg $chan Welcome $nick to this channel.
}

If you need a restriction only for members with access level 4 and higher, then like this:
Code
on +4:JOIN:#channel:{
  .msg $chan Welcome $nick to this channel.
}


You can find a more detailed description here -> https://en.wikichip.org/wiki/mirc/access_levels



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