The investors will have to guarantee the financing personally.
A partnership is far weaker than even an LLC. I'm unaware of anyone willing to lend to a partnership. As for an LLC, it's possible to borrow in the name of the LLC if the partners agree to personally guarantee the loan. But lenders are very wary of lending money to LLCs unless they have a long history and are well established.
Besides, the investors shouldn't be going into it in a partnership structure in any case. Check with a lawyer for more information on that.
One other possibility--if the investors have money in IRAs--is to transfer some or all of their funds to self-directed IRAs. Then they can use that money for real estate investments. But it has to be done right.