It is Sunnah for the people who are already sitting to make space for a newcomer by moving up. It is stated by Sayyiduna Waasilah Bin Khattab RadiAllahu Ta'la Anhu that a person came to the Noblest Prophet ﷺ. He ﷺ moved up (i.e. changed his position making space) for him. The person said 'Ya RasulAllah ﷺ there is already enough space. (Why did you take the trouble of making more space?)' The Prophet of Rahmah ﷺ replied, "The right of a Muslim is, when his brother sees him, he should make space.' ( Shu'ab-ul-Iman, vol. 6, pp. 468, Hadees 8933)