What happens when you are assessed by a mental health crisis team at the hospital emergency room? Do they lock you up?

Assessment. The emergency mental health assessment includes an evaluation of whether the patient is in immediate danger of self harm or harm to others. Usually this leads to a recommendation for any needed treatment which the patient and family may accept. In cases of severe threat to self or others, the team does have the ability to put a patient on emergency hold for more careful evaluation (for 72 hrs).
State's rights. Each state has its own laws governing mental health evaluation during a crisis. Generally speaking the mental health professional and/or crisis team will evaluate the individual for possible harm to self and possible harm to others. Depending on the criteria in the mental health law(s) in your state, if the person meets the criteria, they may be involuntarily committed.
No. A mental health crisis team's job is to evaluate you and determine the right treatment and level of care you need. They are there to ensure your safety and make recommendations based on that. They may recommend a variety of things ranging from psychiatric hospitalization to discharge home with follow up with a/ your psychiatrist.