It's in the manual. Chapter 5. Use the HOST_GROUP option:
HOST_GROUP
HOST_GROUP group_name host_list
Defines a group of hosts for use in INCLUDE, INCLUDEALL, EXCLUDE, EXCLUDEALL, and RESERVE option lines. Multiple HOST_GROUP lines add all the specified hosts into the group.
group_name: Name of the group being defined. Host group names are case sensitive.
host_list: List of host names in that group. Names are case sensitive. Set the GROUPCASEINSENSITIVE options file keyword to turn on case insensitivity. See "GROUPCASEINSENSITIVE."
To define the host group "Pacific" consisting of "tokyo," "seattle," and "auckland":
HOST_GROUP Pacific tokyo seattle auckland
Anywhere a host name can be used in an options file, an IP-address can be used instead.
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