Understanding how one builds a physics identity is an important step to creating structures and practices that support physics students both in and out of the classroom. We have investigated how university students’ physics identities can be fostered through teaching youth in informal physics programs. We collected reflections and interviews from participants in three different informal programs and analyzed them with an operationalized Communities of Practice framework. Our analysis shows that students’ identities can be supported through different structures and practices within each program. We find that students’ personal values aligning with the program’s mission was the biggest predictor of membership, while interactions with members of the community served as the most important mechanism for integration into the community. This work illuminates the specific aspects of informal physics programs that facilitate physics identity formation among the university students who choose to participate.