Citizen Social Science (CSS), defined as participatory research co-designed and directly driven by citizen groups united by a shared social concern, is closely linked to the challenge of engaging “hard-to-reach” populations. These shared concerns often reflect underlying social inequalities or vulnerabilities, which may stem from socioeconomic conditions in specific neighborhoods, shared health issues, or the marginalization of groups based on age, gender, or migrant status. This presentation examines practical approaches developed and implemented since 2012 by the OpenSystems research group, based in Barcelona. Over the years, the group has collaborated with several youth communities that are often underrepresented in citizen science initiatives, including students in segregated schools, participants in special education programs, residents of sheltered homes, and young people engaged in NGO-led social programs. Successful approaches, based on the specificities of each neighborhood and on potentiating the youths’ soft skills will be discussed, together with common challenges. Other projects engaged adults whose voices are often underrepresented, including people with mental health conditions and their families, who participated in a CSS initiative on social support networks. Further initiatives involved women in prison and migrant communities. In these cases, participants’ lived experiences was placed at the center of the research, which was conducted in trusted community spaces designed to be welcoming and inclusive. The main challenges encountered were linked to the difficulty of creating and sustaining connections with the communities.