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 will tell his friends, which mean more than one friend, so he can incorporate all the entries ?
Better to ask his friends to make an account on bitcointalk and submit their entries and this may be helpful in future contests also.
You can compete in two different categories, so, technically, you can ask two of your friends to try to make something for this competition. But is not that easy as it seems, I must tell you. I know several people who are good at drawing, and all of them were saying the same thing: you have to spend a lot of time to make something re presentable, and since the award is not guaranteed, they don't want to risk. This makes the good submissions to the Art contest even more valuable. There are really some great works in the thread, Kudos to you guys!