As you know a weblogic domain consists of set of servers running independently or in a cluster mode, sharing the distributed resources. And in most environments weblogic  cluster consists of multiple managed servers running simultaneously and working together to provide increased scalability and reliability.  These servers can run on the same machine, or be located on different machines. 
It's a common task to increase a cluster's capacity by adding new machines to the cluster to host the new server instances.  You can do it by manually installing weblogic binaries to the new host and use pack/unpack commands to add a managed server to this new host.  But with Enterprise Manager Grid Control 11gR1 (EMGC) there is  another way - Fusion Middleware Domain Scale Up  procedure. I'm going to show you how it works.

Here is a picture of  my medrec_oradb weblogic domain, what is registered in EMGC. It contains an admin server and a cluster MedRecCluster with  the single managed server MS1. Both admin and managed servers are on the same host oel46-vmware, it's a virtual machine with OEL 4.6 that runs inside our Oracle VM infrastructure. 



And here are the application deployments, note that couple of applications are deployed to the cluster.
First of all I have to prepare a new machine that will host new managed sever of my cluster. I created new VM with OEL 5.4 using the corresponding Oracle VM template available in Oracle E-Delivery site for Oracle Linux and Oracle VM and named it wls1032.
Next step is to install Oracle EM Grid Control 11gR1 Agent to this new host.  You can download it from the OTN page and install it manually,  or you can use Agent Installation Deployment procedure available in EMGC  (Deployments->Agent Installation->Install Agent). Anyway, when you agent is up and running on the new machine, you will see it in EMGC Console in the Targets->Hosts subtab.

Now we are ready to scale up our weblogic domain. Click the Deployments tab in Oracle Enterprise Manager Grid Control, and then click Deployment Procedure. Select a Fusion Middleware Domain Scale Up procedure from the list, and click Schedule Deployment. The first page of the FMW Domain Scale Up Wizard is displayed and you can proceed with the deployment process.
Select the domain from list, enter the working directory on the admin server host, and also fill the weblogic credentials for the administration server console and the OS credentials for the  admin server host.  Click Next button. 

The next step allows you to configure you domain, to add a new manager server to the cluster you should select the cluster in the tree and click Add Server button. Select the newly added server in a tree, choose the target host and  enter the configuration details of your managed server. You can also add new machine and node manager details.  Please note that you cannot change the values in  Domain Location and Fusion Middleware Home fields, so these locations on the target host will be the same as for the admin server host.   Working directory on the target host should have enough free space to store FMW home binaries and domain configuration files.  In my experience the working directories should have at least 3 Gb of free space.  The last thing you should fill is the OS credentials for the target host.