Greystar Real Estate Partners, the Charleston company that already managed more apartments than anyone else in the country, has bought another big apartment manager.

Greystar said today that it has acquired Riverstone Residential Group, which it described as a leading manager of apartments in the United States. The seller is CAS Capital Ltd., a London-based, wholly owned subsidiary of Regis Group PLC, which is owned by Nick and Peter Gould, according to Greystar’s announcement. Greystar did not disclose a price.

Together, the companies manage more than 385,000 apartment units in major markets in the United States and abroad and have more than 10,000 employees, Greystar said.

That is a big jump from Greystar’s own recent figures. Last October, the company said it managed about 215,000 apartments. Last August, it said it had more than 5,000 employees.

“Riverstone is one of the nation’s most highly respected multifamily firms and we are proud to have them join our Greystar family,” said Bob Faith, Greystar’s chairman and CEO. “Combining forces makes us even stronger, especially in local markets where we are bringing together some of the most talented and experienced multifamily professionals in the industry.”

Stronger in the West

Greystar said the deal will greatly expand its local presence in major markets throughout the country, particularly in the western United States.

Faith said the company’s local teams will now “harness the power of our combined technology and service platforms” to offer more services to clients, partners and residents. The combined company manages apartment communities for many of the industry’s leading ownership groups and institutions, according to the announcement.

Greystar has been among the largest apartment managers in the country for a number of years, the company said. Last August, it said the National Multi Housing Council, an apartment industry group, had ranked it the largest for three years running.

Built by buying

Acquisitions are how Greystar got its start. Faith formed the company in 1993 by buying and combining a series of Texas-based apartment service companies, Greystar said last August, when it marked its 20th anniversary. The company was based in Houston until 1999, when it moved its headquarters to Charleston. Its offices are at 18 Broad St., according to its filings with the Securities and Exchange Commission.

Over its 20-year history, Greystar has absorbed the management arms of several major operators, including JPI, Archon and Glacier, the company said.

Managing apartments for other owners is only part of the business. Greystar also develops apartments, with $1.5 billion of development under way, and runs an investment management business with more than $7.5 billion in assets, according to the announcement.

It has also been growing abroad. Greystar said it recently expanded into Mexico and the United Kingdom. Its first British purchase, made with Goldman Sachs and closed last October, was a portfolio of 21 student housing properties with more than 6,900 beds, mainly in London, Manchester, Leeds and Liverpool. The properties were bought out of the bankruptcy administration of Opal Property Group, Greystar said at the time.

Who is selling

Nick and Peter Gould have led Regis Group for 30 years, investing in residential real estate in the United States, Britain and France, according to the announcement.

The Goulds are founding partners of Invitation Homes, which was established in 2012 in partnership with The Blackstone Group. Invitation Homes has spent more than $8 billion to buy more than 43,000 houses to rent out in the United States and is now the country’s largest owner of single-family rental homes, the announcement said. B2R Finance, a joint venture of the Goulds and Blackstone, makes buy-to-rent mortgages for residential property investors.

“We are excited to complete this transaction with Greystar,” said Nick Gould, chairman of the board of CAS Capital. “The combining of these two great companies represents the realization of our vision to create the highest quality multifamily residential service provider in the U.S.”

Peter Gould, the vice chairman, called Greystar “a great strategic and cultural fit for the company.” He said the Goulds “now entrust a great company to Bob Faith” as they turn to building their other large businesses.