Here is what is stated in the FAQ:
14.  Do Supplementary Cards receive complimentary access to Plaza Premium Lounge?
  Yes, complimentary visits to Plaza Premium Lounge are valid for Principal and Supplementary Cards.
   
15.  Do Supplementary Cardholders receive a complimentary Priority Pass membership for global airport lounge access?
  No, the complimentary Priority Pass membership is only valid for Principal Cardmembers.
   
16. Am I allowed to bring accompanying guests into the lounge?
  For use of Priority Pass at global lounges – Accompanying guests are allowed into the lounge, but each guest will count as a single visit from your complimentary Priority Pass access or will be charged RM90 if there are no complimentary access remaining.

For use of your Citibank PremierMiles Card at Plaza Premium Lounge – Accompanying guests will be entitled to a 20% discount on the prevailing lounge entry rate for a 2-hours package.
p/s: btw, LCCT lounge is no longer available for complimentary access using Premier Miles. It was constantly mentioned in the previous few pages.

Sorry. Didn't notice you were asking about Citi plat. I thought it was premier miles.
Anyway, am not sure about the Citi plat. But if miles allows supp, probably plat also allows. You can always call in Citi or write them an email to clarify.

If you are referring to our local lounges, even for RM50 it is not worth it.