Currently filtering by host group and its nested host groups is possible in frontend by appending /* to the host group. For example if there are groups
server/mysql, server/oracle then to include both groups server/* has to be specified.

It's not obvious, is only partially supported by frontend and not supported by server at all.

To improve it frontend and server should automatically assume that all nested groups are included if simply the top group name is given (server in the above example).