"Entered" means you signed up for the contest. There is no mention of what happened after that. "Joined" is not really clear about the extent of your involvement in the contest.
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tweetykrEnter means in my context 'signs up' and its meaning doesn't imply participate or join.There is no reason to enter a contest unless you wish to participate. Entering any competition is the first step of participating, so entering does imply participation. By entering you declare yourself to be a contestant, a participant.