We investigate the development of expert identities through the use of the sociocultural perspective of learning as participating in a community of practice. An ethnographic case study of biophysics graduate students focuses on the experiences the students have in their research group meetings. The analysis illustrates how the communities of practice-based identity constructs of competencies characterize student expert membership. A microanalysis of speech, sound, tones, and gestures in video data characterize students' social competencies in the physics community of practice. Results provide evidence that students at different stages of their individual projects have opportunities to develop social competencies such as mutual engagement, negotiability of the repertoire, and accountability to the enterprises as they interact with group members. The biophysics research group purposefully designed a learning trajectory including conducting research and writing it for publication in the larger community of practice as a pathway to expertise. The students of the research group learn to become socially competent as specific experts of their project topic and methodology, ensuring acceptance, agency, and membership in their community of practice. This work expands research on physics expertise beyond the cognitive realm and has implications for how to design graduate learning experiences to promote expert identity development.