I posted this question on the isaserver.org forums but had no responses:

I have created a TMG array (standalone, with an array manager) but are having some problems getting the first member to join. I have added the server to the ‘managed server computers’ group however this doesn’t seem to show up in the configuration page as per the guide http://technet.microsoft.com/en-us/library/dd440981.aspx#join. Is there something I’m missing?

As there were no responses I can only assume the information is not well known.

Verify that all array members are included in the firewall policy. Click the Monitoring node, then click the Configuration tab, and verify that all the array members are listed under Configuration Status.

Despite the Microsoft advice suggesting that the newly added server will show up on configuration before the adjacency is established, this doesn’t appear to be the case. It seems the array members will only show up under the configuration section when you have joined the array manager.

Monitoring Configuration