Hi CF,
For each new Subscriber a clusterwide reboot is required so a maintenance window would be your best bet;
Adding New Subscriber Servers to the Cluster After the Installation
If you need to add servers to the cluster, perform the following tasks:
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Caution If you changed any NT Service passwords on the servers in the cluster, you must use the password utility to reset the passwords to the default by using the Admin Utility. The installation fails if you do not to perform this task.
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1. Install the operating system on the subscriber server(s) by using the Cisco-provided operating system disks and the operating system documentation.
2. Verify that you configured name resolution, for example, DNS, before you install Cisco CallManager.
3. Install Cisco CallManager on the subscriber server(s) by using the Cisco CallManager Installation, Upgrade, and Recovery Disks 1 and 2.
Make sure that you enter the publisher database server name correctly, so authentication to the publisher database server can occur.
4. Upgrade to the version of Cisco CallManager that currently runs in the cluster.
5. Install the same Cisco CallManager service release and operating system hotfixes/service patch that you have applied to the existing cluster.
6. **Reboot all nodes in the cluster.**
From this doc;
http://www.cisco.com/en/US/customer/products/sw/voicesw/ps556/prod_installation_guide09186a00803f5c57.html#wp534948
Hope this helps!
Rob
