Your license determines whether or not clustering is enabled and how many nodes are supported. To check on the number of clustered servers your license allows, see the license information after logging into the application administration tool.
Next, navigate to System > Settings > Caches. Toggle Clustering to Enabled and then Save settings (note that the Clustering option will only be displayed if your license allows it). The server will automatically discover and join the cluster. You'll then see additional options in the cache panel to monitor cluster activity.