You can join a system to a Linked Mode group during the vCenter Server installation or after you install vCenter Server.
For example, suppose you have three machines on which you want to install vCenter Server. You want the three instances to be members of a Linked Mode group.
On Machine 1, you install vCenter Server as a standalone instance because you do not have a remote vCenter Server machine to join.
On Machine 2, you install vCenter Server, choose to join a Linked Mode group, and provide the fully qualified domain name of Machine 1.
On Machine 3, you upgrade to vCenter Server 5.x. After the upgrade, you configure Machine 3 to join either Machine 1 or Machine 2. Machine 1, Machine 2, and Machine 3 are now members of a Linked Mode group.