‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) said: "No man should move another from the place he is sitting then sit in it. Instead, make room and spread out."
This Hadīth includes two of the etiquettes related to gatherings: 1. It is not permissible for one person to make another leave the place he is sitting in - which he got to before him - and then sit in it. 2. It is obligatory for those who are present to spread out to find a place for a newcomer among them. Allah, the Exalted, says (what means): {O you who believe! When you are told to make room in the assemblies, spread out and make room and Allah will give you ample room.} [Sūrat al-Mujādilah: 11]