Explanation:

In the Administering Windows Server Hybrid Core Infrastructure guidance for Azure File Sync, the workflow to onboard an additional Windows Server to an existing sync topology is explicit. Once a Storage Sync Service, sync group, and cloud endpoint (the Azure file share) already exist and are syncing with another server (Server1 in this case), you only need to prepare and join the new server (Server2) to that configuration.
The documented sequence is: (1) install the Azure File Sync agent on the Windows Server you want to add; (2) use the agent to register the server with the correct Storage Sync Service resource; and (3) in the existing sync group, add a server endpoint that points to the local path on the newly registered server. The materials emphasize that creating a new Storage Sync Service or adding another cloud endpoint is unnecessary when the Azure file share is already in the sync group. The "server endpoint" binds the on-premises path to the already defined cloud endpoint in the sync group, enabling bi-directional sync. Therefore, the correct order is:
Install agent on Server2 # Register Server2 with the Storage Sync Service # Add a server endpoint to the sync group.