Elon Musk plays many roles with President-elect Donald J. Trump. He is Mr. Trump’s most important donor, most influential social media promoter and a key adviser on policy and personnel.

For most of the time since Election Day, he has also been Mr. Trump’s tenant.

Mr. Musk has been using one of the cottages available for rent on Mr. Trump’s property at Mar-a-Lago, the former Marjorie Merriweather Post home in Florida that Mr. Trump converted into a members-only club and hotel in the 1990s, according to two people with knowledge of the arrangement. The cottage where he has been staying, named Banyan, is several hundred feet away from the main house, according to a person who knows the property.

Staying right on the grounds has helped provide Mr. Musk with easy access to Mr. Trump.

He can drop in on Mr. Trump’s dinners, such as one he had recently with Mr. Musk’s rival, the Amazon founder Jeff Bezos.

Mr. Musk, who spent more than a quarter of a billion dollars in the final months of this year’s election cycle to help elect Mr. Trump, has attended personnel meetings in the Mar-a-Lago Teahouse, sat in on phone calls with foreign leaders and spent hours with Mr. Trump in his office. Mr. Musk’s employees from his various businesses have also been integrally involved in the transition, vetting prospective candidates for senior administration jobs, in interviews at the Trump transition headquarters in West Palm Beach.

Mr. Musk is not the only member of the president-elect’s inner circle who has been bunking on Mr. Trump’s property. Vice President-elect JD Vance has stayed in one of the cottages at Mar-a-Lago when he has been in Palm Beach and has been there frequently during the transition, according to a person with knowledge of his stays. And others are said to have used cottages since Election Day. But few have been as omnipresent as Mr. Musk.