When someone "came on board," it typically means that they have joined a team or organization. However, there are numerous synonyms for this phrase that can be used to add variation to written or spoken communication. Some of these synonyms include "joined," "signed on," "enlisted," "became a member," "started working with," "got involved," "jumped aboard," "joined up," "got on the train," and "hitched a ride." Choosing different synonyms can help to keep communication fresh, and prevent repetition or boredom. Consider using some of these variations in your next written or spoken communication to keep things interesting and varied.