enter in(to) (something)
1. To move into something. Once you enter into the parking garage, turn left.
2. To agree to something, such as a contract. It is common law that those who enter into a written agreement must adhere to the terms contained therein.
3. To sign up or enlist for something. I'm a pretty good singer, so I'm definitely entering in the talent show this year.
4. To sign someone else up for something. In this usage, a noun or pronoun can be used between "enter" and "in" or "into." I'm definitely entering Sasha into the talent show this year. She may be shy, but she's a great singer.
5. To become a part of or factor in something. Once my uncle's shortcomings entered into the dinner conversation, I knew that a fight was inevitable. If Mom and Dad think that alcohol will enter into the equation, they definitely won't let you go to that party.
6. To join or begin to participate in something. Did you hear that Jody is entering into med school in the fall?
enter someone or something in(to) something
to enroll someone or something in something; to make someone or something a competitor in something. I will enter you into the contest whether you like it or not. The trainer entered his fastest horse in the race.
enter in something
to enroll as a participant in something, such as a contest, competition, etc. She was not ready to enter in the competition. I can't enter in that contest. I'm not prepared.