The information provided on "Hosts and host groups" page is scant and, therefore, needs to be updated.
Although, there is a link leading to the page on how to configure a host, it could be useful to add a note that "a host must belong to at least one host group. A new group can be created and linked to the host group by adding a non-existing group name."
Suggested in typo reports: This should explain what host groups are and how they can be used. There's a lot of functionality here which links user access to hosts, notifications, maintenance, etc. which can be confusing for users to figure out on their own.