They can always enter, but try these things first.
1) Make sure you've added the event (Tournaments tab > add a tournament)
2) Make sure the rounds are in (if yours looks like the picture below, you didn't complete this process when you added the event). Click on the green + symbol and add the rounds
3) Make sure the players are added to the event (click on 'add players')
Remember, they can always add stats even if you forgot the above three steps. Check out the video below to see how players can do this