Background: The patient portal is a widely available secure digital platform offered by care delivery organizations that enables patients to communicate electronically with clinicians and manage their care. Many organizations allow patients to authorize family members or friends—“care partners”—to share access to patient portal accounts, thus enabling care partners to receive their own identity credentials. Shared access facilitates trilateral information exchange among patients, clinicians, and care partners; however, uptake and awareness of this functionality are limited. Objective: We partnered with 3 health care organizations to co-design an initiative that aimed to increase shared access registration and use and that can be implemented using existing patient portals. Methods: In 2020, we undertook a rigorous selection process to identify 3 geographically diverse health care organizations that had engaged medical informatics teams and clinical champions within service delivery lines caring for older adults. We prioritized selecting organizations that serve racially and socioeconomically diverse populations and possess sophisticated reporting capabilities, a stable patient portal platform, a sufficient volume of older adult patients, and active patient and family advisory councils. Along with patients and care partners, clinicians, staff, and other stakeholders, the study team co-designed an initiative to increase the uptake of shared access guided by either an iterative, human-centered design process or rapid assessment procedures of stakeholders’ inputs. Results: Between February 2020 and April 2022, 73 stakeholder engagements were conducted with patients and care partners, clinicians and clinic staff, medical informatics teams, marketing and communications staff, and administrators, as well as with funders and thought leaders. We collected insights regarding (1) barriers to awareness, registration, and use of shared access; (2) features of consumer-facing educational materials to address identified barriers; (3) features of clinician- and staff-facing materials to address identified barriers; and (4) approaches to fit the initiative into current workflows. Using these inputs iteratively via a human-centered design process, we produced brochures and posters, co-designed organization-specific web pages detailing shared access registration processes, and developed clinician and staff talking points about shared access and staff tip sheets that outline shared access registration steps. Educational materials emphasized the slogan “People remember less than half of what their doctors say,” which was selected from 9 candidate alternatives as resonating best with the full range of the initiative’s stakeholders. The materials were accompanied by implementation toolkits specifying and reinforcing workflows involving both in-person and telehealth visits. Conclusions: Meaningful and authentic stakeholder engagement allowed our deliberate, iterative, and human-centered co-design aimed at increasing the use of shared access. Our initiative has been launched as a part of a 12-month demonstration that will include quantitative and qualitative analysis of registration and use of shared access. Educational materials are publicly available at Coalition for Care Partners.