The "Agent Host Name" agent attribute is not available in the "Select Agent Attribute" drop-down on the "Create New Agent Group" page within the Enforce console. However it is listed on the "Agent Attributes" page in Enforce which makes it appear that it can be used as a condition for an agent group. The "Create New Agent Group" page is used to add new agent groups and you have the option of basing the agent group membership on either agent attributes or endpoint server.

The "Agent Host Name" agent attribute is not available in the "Select Agent Attribute" drop-down because it is intended that the "Always include these agents" field will be used for that purpose. You can add a list of agent host names to that field, one per line. This list of agents will always be added to the agent group you are creating.