A Group Resource is a binary file with a format described by the following C structure and constants:

#define GROUP_RESTYPE       'G'
#define GROUP_RESVERS       '1'

typedef struct GroupResource {
    //  ----------------------------------------------------------
    uint8_t         resType;            // (0x00) Resource type (Group = 'G')
    uint8_t         resVers;            // (0x01) Resource version (Group = '1')
    uint16_t        maskBytes;          // (0x02) Number of bytes in mask
    uint8_t         reserved1[12];      // (0x04) -
    //  ----------------------------------------------------------
    uint8_t         variableParams[];   // (0x10) Beginning of variable parameters
//  uint8_t         mask[*];            // (????) Bitmask (size is maskBytes)
//  char            name[*];            // (????) Group Name (c-string)
    //  ----------------------------------------------------------
} GroupResource;