Code:
; enter the mask-to-match here
alias -l watchmask { return *!user-mask*with@*wildcards.allowed }

; this is the join event; change #channel and the echo part to whatever you like 
on !*:join:#channel: {
  if ($watchmask iswm $fulladdress) { echo -a $nick joined $chan on $network with $fulladdress matching $watchmask }
}