Riamus2, thanks for breaking his explanation down into a thorough and understandable one.

I suppose we can also use $regsubex to achieve the same outcome:

Code:
on !*:join:#yourchannel:{
  msg $left($regsubex($str($chr(44),$ulist(*,admin,0)),//g,$ulist(*,admin,\n)),-1) Your Message Here.
}