This is a reference to Jaʿfar al-Ṣādiq attending the Friday prayer with other Muslims, in contrast to some Muslims who did not pray with anyone who did not share their doctrinal tendencies. Hossein Modarressi cites this reference in his discussion of some of the features of al-Ṣādiq's rejection of sectarianism.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).