By Konrad Putzier and Eliot Brown | Updated March 12, 2019 9:38 a.m. ET

Five months after the co-working firm Bond Collective signed a 42,000-square-foot lease in a Brooklyn office building, the property’s owners flipped it for a hefty profit.

“Hey, we can essentially do the same thing,” Bond Collective founder Shlomo Silber recalled thinking after learning of the sale.

Four years later, the firm is co-managing its own real-estate fund. The fund owns stakes in properties in Miami, Nashville, Chicago and New York City and is in contract to buy two more.

Bond Collective is one of several real-estate startups that has tweaked its original business model in hopes of boosting revenue and creating new opportunities.

For their main business, these firms sign long-term leases or management agreements, turn the space into furnished offices, apartments or hotel rooms, then rent it out to tenants under flexible terms.

Now, several of these property companies are also raising money to launch real-estate investment funds. That includes the co-working giant WeWork Cos., which recently rebranded as the We Company. It has one of the biggest of these funds, having raised $745.4 million from investors as of March 8, securities filings show.

The startups tend to rely primarily—or even entirely—on outside investors to put up the cash for the funds. But the property firms usually follow a private-equity model of collecting a management fee or getting a stake in the buildings they acquire.

The strategy could boost profits and growth. When buying property, these firms may also get access to buildings they wouldn’t get otherwise. And because these firms invest little if any of their own money, the financial exposure is minimal.

But these investment funds bring other risks and potential conflicts, especially at times when the interests of the main business and the fund diverge, real-estate analysts and attorneys warn.

For instance, if a co-working company defaults on its lease and an affiliated investment fund is the property’s owner, the two entities could be pitted against each other.

“This could, in a period of economic stress on that company, create difficult conflicts to deal with,” said Minta Kay, a partner and chair of the real-estate industry group at law firm Goodwin Procter LLP.

Moreover, big landlords, already skittish about the rising clout of co-working companies, could become more reluctant to partner with startups if they perceive them as rival investors.

“I think that you kinda gotta be one or the other,” said Bryan Murphy, CEO of flexible-office and meeting-room operator Breather, which has shied away from real-estate acquisitions. “You’re either a competitor to landlords or a partner to landlords.”

Some of the benefits and complications from investing in property were on display with the marquee acquisition made by WeWork.

WeWork Property Advisors, the fund it runs with private-equity firm Rhone Group, agreed a year and a half ago to buy the Lord & Taylor building in Midtown Manhattan for $850 million, with plans to spend another $400 million on a redevelopment. It was a marquee deal that raised the company’s profile as a major real-estate investor.

But the high-profile purchase also put new strain on WeWork. After failing to attract other equity partners, the fund borrowed $950 million, and the building’s seller, Hudson’s Bay, kept a $125 million stake in the property. WeWork also invested some of its own money alongside fund investors.

WeWork assumed other risks. It agreed to lease the entire property for $105 a square foot for 20 years, giving a corporate guarantee for 15 years of rental payments, according to people familiar with the deal. That is a high rent and an unusual structure for WeWork, which typically pushes back against landlord requests for corporate guarantees and prefers to agree to around a year of guaranteed payments. The complicated deal took more than a year to close.

Co-living firms, which rent out furnished rooms in shared, serviced apartments, are getting into the investment game, too. Germany’s Medici Living GmbH partnered with property investment firm Corestate Capital Holding SA to invest around $1.1 billion in apartment developments in Europe and with Ralph Winter’s family office W5 Group to invest $300 million in U.S. projects. San Francisco-based co-living firm Starcity said it is raising a $1 billion-plus fund.

Sonder, the apartment-hotel company that manages short-term rental units, said it is exploring a real-estate investment fund for as early as this year.

Brad Hargreaves, founder of the co-living company Common Living Inc., said his investment fund improves his access to the best properties. The company announced last month its new investment vehicle, Common Strategic Capital, in partnership with Mexican multifamily owners Sal and Alberto Smeke.

“Great residential locations don’t come up for sale at decent prices that often,” Mr. Hargreaves said. “So when you see one, you want to be able to jump on it.”