Social costs and limited reach inhibit our use of social capital to solicit help. However, individuals are not the only holders of social capital: groups also possess reputations and social capital, and are often prepared to vouch for their own members. In this paper, we design methods for mobilizing this collective social capital in sociotechnical systems, enabling an individual to ask a trusted group whether it is willing to invest its reputation in doing them a favor. We instantiate this concept with Founder Center, a web platform in which members of a local entrepreneurship accelerator ask the accelerator community to collectively make them introductions to potential funders. In a field experiment, enabling access to collective social capital in this community nearly doubled the odds of members making a social capital request. Requests fulfilled utilizing collective social capital were at least as effective as ones utilizing traditional interpersonal social capital.