Volunteer motivation has been researched in HCI in several application domains. However, motivational complexities of digital volunteerism in science-based domains, such as medical research are under-explored, especially when involving volunteers with marginalized identities. We aim to study the socio-technical complexities of voluntary participation in a digital science-based research platform and propose recommendations to enhance volunteer motivation. We describe a survey study of volunteers on Step-Up for Dementia Research platform (n = 266) to capture nuances in their demographics, experiences, motivations, well-being, and psychological needs. Our findings reveal the features that facilitate or impede sustainable volunteer participation and the 5 identities based on which volunteers derive meaning from their work. We propose 8 recommendations to navigate the digital divide and foster inclusion, build wider participation by engaging with the social construction of volunteering and focusing on prosocial values, and enhance volunteer well-being by fulfilling their cognitive, emotional, and psychological needs.