Borrowing from chadbdurham's regex with a little modification, you can simply do this:

on $*:TEXT:/^[!@.~';](join|part)\s(\x23\w+)/Si:*: {
  if ($regml(1) == join) { join $regml(2) | .amsg $me has joined $regml(2) }
  if ($regml(1) == part) { part $regml(2) | .amsg $me has left $regml(2) }
}

Usage: choose any symbol for the join or part trigger and then do, for example:
@join #channel
@part #channel

To make it workable for your own nick and ops or owners etc... only:

on $*:TEXT:/^[!@.~';](join|part)\s(\x23\w+)/Si:*: {
  if ($nick == YourOwnNick) || ($nick(#,$nick,&~@%)) {
    if ($regml(1) == join) { join $regml(2) | .amsg $me has joined $regml(2) }
    if ($regml(1) == part) { part $regml(2) | .amsg $me has left $regml(2) }
  }
}

And I think using /amsg is fitting.