This command is exactly like GEORADIUS with the sole difference that instead of taking, as the center of the area to query, a longitude and latitude value, it takes the name of a member already existing inside the geospatial index represented by the sorted set.
The position of the specified member is used as the center of the query.
Please check the example below and the GEORADIUS documentation for more information about the command and its options.
(integer) 1redis> GEOADD Sicily 13.361389 38.115556 "Palermo" 15.087269 37.502669 "Catania"
(integer) 2redis> GEORADIUSBYMEMBER Sicily Agrigento 100 km
1) "Agrigento" 2) "Palermo"