attach a script to the trigger
; _caller refers to the person who triggers
_caller = THISLIST 0
;leader of the person who triggers will sidechat
LEADER _caller SIDECHAT "blah"
but i think SIDECHAT is not a public command so in MP, the message will not be displayed for every player. i remember seeing a thread with the solution to this prob though so if you need to you can search this forum for it