The contest is done regularly and is proven to deliver the price so it is not a surprised that it attracts a lot more participants. I even share the contest to my friends who are not a member of this forum. They got interested but I wonder if they will be able to submit their work 
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You can submit on your friends behalf as you have account on forum but you also need to have Roobet account to mention your username on the casino as funds are transferred to your casino account.You can mention that you have submitted artwork on their behalf or they can register on forum as there is no restriction for newbies I think..Isn't that one person submit one entry ?
Yeah he can send on his friend behalf but then if he wants to participate himself in the artwork then he may not be able to do so
Secondly he told that he wil tell his friends, which mean more than one friend, so he can incorporate all the entries ?
Better to ask his friends to ae an account bitcointalk and submit their entries.