1: I'll advise my friend to atteent the rehearsal because it is very important to dance  confidently in front of others.
2: it is a opportunity to correct our mistakes.
3: without one person the whole group will be affected.
4: it makes us more confident.

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Tell him that because of him not attending the practice, the performance is incomplete. Try flattering him by saying that he plays the most important role in the act, and without him, it's not possible. Tell him that his confidence will boost if he practices. Say that you're having a lot of fun at the practice, and you want him to join in as well. At last, try saying that this competition is very important for you, and you need to win.
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